spring? And why is it that parent and offspring are never exactly alike? Mendel, during the 19th century, performed the first systematic experiments, which have helped very materially in answering these questions. Mendel, fixing his attention on a single character at a time, crossed garden-pea plants that differed from each other. Tall ones were crossed with short, greenseeded ones with yellow-seeded, wrinkled ones with smooth and so on. The untrained observer would expect that when two individuals unlike in some respect are mated, the offspring would show characteristics that are somewhere between the characters of the parents. Analogous to this is the partially correct view that children resemble both parents. Most people fail to note that children have some characters just like those of the father and others similar to those of the mother.

For eight years, Mendel experimented with simple garden peas. At first he used tall and dwarf peas of pure strains, crossing them artificially and noting the results in several subsequent generations. In the first generation he found all the offspring were of the tall variety like the first parent. As he found no dwarfs, he concluded that one of the parents, the tall pea, contained some factor within it which might be said to be dominant over corresponding factors in the dwarf pea. This conclusion has been substantiated by subsequent experiments. On crossing the offspring of the tall and short peas, he found these offspring to be in the ratio of three tall to one dwarf. In other words, he found that the dominant element reappeared in the second filial generation in approximately the ratio of three to one. Mendel next concluded that in addition to the dominant factor there must be a recessive factor which tended to come into dominance in approximately one out of three of the offspring in the second generation. The recessive character is really latent as it appears in later generations. Plants having the recessive character breed thereafter, true, but the dominants will repeat the history of the first generation in two cases and become pure recessive thereafter in the third case.

Thus, Mendel found the complete resemblance of the offspring to one of the parents was quite the rule with each of his pairs of characters. Since his time, however, various experiments have indicated that some pairs of characters do not show complete dominance. His experiments further show that hybrid offspring cannot reproduce itself in offspring all having the same characteristics.

It should be borne in mind that every organism is made up

of many determiners. These determiners are found in greater number among the higher animals than in the case of the lower. From the many experiments that have been made, the conclusion may be drawn that each pair of alternative characters behaves according to the above principles regardless of other characters present. The law of unit characters, as it is called, helps one to understand the great diversity among plants and animals. The greater the number of characters that make up a given species, the greater is the possible number of combinations, and the smaller is the chance that any particular combination will occur again. The unit characters are passed from generation to generation without apparent alteration, and in such a way that an animal or plant either has or has not the character. Mendelian unit-characters in human beings include such traits as the color of the hair, the color of the iris (eyes), night blindness, color blindness, deafmutism, certain kinds of feeble-mindedness, and extra digits.

If two parents differ in one contrasted character (Mendelian) only, e. g., color of skin, grandchildren may be of two or possibly three kinds, like the grandfather or like the grandmother, and possibly a sort of compromise between them, that is, imperfect dominants. If the two parents differ in two pairs of contrasted Mendelian characters there may be four different types among the grandchildren.

Professor Conklin writes: 2

"When there are five such pairs of contrasting characters in the parents, there may be (2) or 32 types of grandchildren showing various combinations of these five characters; when there are ten pairs of contrasting characters there may be (2)10, or 1,024 types of grandchildren. Between different races there are many more than ten unit differences, and thus with a relatively small number of mutant characters an enormous number of different combinations of the characters is possible in the offspring."

The Bearers of the Heredity.-Corresponding to the unit characters mentioned above are certain minute bodies in the germ cells which are responsible for the transmission of the characters from one generation to the next. These are called genes. While they have not been demonstrated microscopically, they are known to be aggregated in certain clearly defined bodies

2 The Direction of Human Evolution (New York, Charles Scribner's Sons, 1921), p. 32.

called chromosomes which can be studied with the aid of the microscope. It has been possible to demonstrate the existence of the genes from observing their effects. The biologists (geneticists) have been able to demonstrate by statistical methods that genes are arranged in the chromosomes in a definite order. It is possible to follow the movements of the chromosomes in cell division in some cases. The minute details of chromosomes can also be seen microscopically in the study of dead cells. At each nuclear division the chromosomes make their appearance, a spindle is formed, and each chromosome splits into two halves, one of which goes to each pole of the spindle. When the cell itself divides, each daughter cell receives exactly the same number of chromosomes as the parent had. When we consider that every time two germ cells unite with one another to form a fertilized egg, ovum and spermatozoon each contributes its share of chromosomes to zygote nucleus, it is obvious that in a very short time the number of chromosomes would be far too large for the nucleus. Nature finds a way of preventing this catastrophe in the process of maturation, which consists primarily in halving the number of chromosomes in each germ cell. As a rule one more division takes place after this reduction (by halving) has been effected. Each immature germ cell contains two sets of chromosomes, one derived from the male and the other from the female. When the reduction division is about to take place the chromosomes pair off, maternal with paternal, and then separate, one of each homologous pair going to each daughter nucleus. It is this separation of the maternal and the paternal that brings about the segregation of the Mendeliam factors.

Thus, for example, if the number of somatic chromosomes is forty-eight in man, and after the pairing of the chromosomes (synapsis), twenty-four pairs of paternal and maternal chromosomes, there are (2) 24 or about twenty million possible types of gametes (mature ova or spermatazoa) in each sex, and since these combine at random at fertilization, the number of possible different types of zygotes (the union cell resulting from sperm and egg) from one parental pair mounts far up in the trillions. Right here is the explanation of variation or individual differences. Variation, then, depends upon how the different chromosomes chance to be distributed or shuffled during the final stages in the formation of germ cells involving chromosome reduction.

For any and every kind of animal or plant, there is a definite

number of chromosomes in each cell, with two exceptions. One is the possibility of the number being slightly different for the two sexes and the other is that the gametes or cells uniting in the process of fertilization contain only half the number characteristic of the species. It has been found that the number of chromosomes differ greatly in different organisms. As the difference in number in each organism is so regular we speak of it as a specific character. Just how many genefactors are carried in each chromosome in man is not known. Without at least an identical chromosome complex people will always be different one from another. The old statement that people are born equal does not apply to biological inheritance.

With the conception and demonstration of the gene the whole subject of heredity has been placed on a concrete basis. The genes are the ultimate elements or the carriers of the unit characters. The answer to the question regarding the possibility of modifying heredity hinges upon the possibility of altering the genes by experimental means. Only the future can give us the

answer.

6. Effects of Long Continued Practice upon Cerebral

Localization

[LASHLEY, K. S., "Studies of Cerebral Function in Learning: The Effects of Long Continued Practice upon Cerebral Localizations," Journal of Comparative Psychology, 1921, Vol. 1, pp. 453-468.] Professor Lashley is one of the leading psychologists engaged in the development of a scientific physiological psychology. His method is experimental, and his conclusions cautious.

The results were obtained by cauterizing selected areas in the cerebrum and cortex of the rat after long continued habit formation. The aim was to discover whether or not automatization consists of the relegation of function from the cortex to the sub-cortical nuclei (from the visual cortex to pulvinar and the external geniculate body). Many theories have claimed such a relegation to be a fact. . . . The experimenter worked on four rats and compared the work done earlier on two other rats. The method consisted in teaching a rat a visual discrimination habit, in a maze of two alleys irregularly darkened and lightened, through 1,300 or more trials in the hope that overlearning would result in automatization. Lesions were then produced in orbital, or temporal, or parietal, or occipital cortical localities by cautery.

After observations twenty-four hours later, concerning the presence of shock, the rat was run through retention test trials, to discover the presence of the habit until the animal had thoroughly relearned the habit. The results point to the following conclusions: (1) Destruction of the visual area results in the loss of the well-formed visual discrimination habit; (2) the retention of other habits proves that the loss of the visual discrimination habit is not the result of operative shock; and (3) long training did not prevent loss of the habit-hence, the function was not transferred, or relegated, from the visual area, to the subcortical nuclei which had been left intact.

7. Is the Theory of Synaptic Resistance Tenable? [LASHLEY, K. S., "Studies in Cerebral Function in Learning," Psychological Review, September, 1924, Vol. 31, pp. 369-375.] (Adapted.)

The theory that the synaptic resistance is reduced by the passage of the nerve impulse has been pretty well accepted. During the past few years, several experiments have been made which seriously question the correctness of the view. Dr. Lashley's experiments show facts contrary to the theory and they indicate that new conditioned reflex paths may be established under conditions where the passage of significant neural impulses over the arcs in question is precluded during the whole course of learning.

[Lashley] blindfolds the left eye of the rat for one month, during which time the rat is trained to avoid the brighter of two lights. When discrimination was perfect with the right eye, the blindfold is transferred to the right eye. As soon as the left eye became adapted, the test showed a perfect discrimination with the eye that was not used. How can this fact be brought into relation with the learning theory? Neural fibers come from the two retina and somewhere impinge upon a common path. In the experiment, the neurones of both eyes prove to be integrated with the avoiding reaction, although only one was subjected to stimulation during training.

We have no evidence that both eyes have synapses in common.

The experiment was repeated with an animal after destroying the visual area of both hemispheres. The results were found to be the same as with the normal rat: an effective performance of the habit with the eye which had been blindfolded during training.