September 15. 192 1] 



NATURE 



87 



factors in linear series. It is surely one of the 

 most remarkable achievements of modern science 

 that we should be able to point to a particular spot 

 in a particular chromosome of a microscopic 

 g-erm-cell and say with confidence that there is 

 something- just there that is responsible for some 

 particular character, such as the colour of the eye, 

 in the adult organism. 



As to the nature of the factors themselves, it 

 seems not unreasonable to conclude that they must 

 consist of definite chemical substances, or, perhaps 

 better, of chemical modifications of living proto- 

 plasm, in the form of minute particles too small 

 to be rendered visible by any means yet dis- 

 covered, but capable of self-multiplication like 

 other protoplasmic units. 



We may further suppose that these factor- 

 forming substances play a part in controlling the 

 development of the organism comparable with that 

 played by magnesium chloride in the case of the 

 developing embryos of the sea-minnow, or bv 

 other chemical substances (hormones) in the 

 normal adult animal. The complex mechanism of 

 mitosis in the division of the cell-nucleus would 

 then appear to be necessary in order to secure the 

 proper distribution of factors throughout the grow- 

 ing body, so that each may reach the particular 

 part that it is destined to influence. 



It must be remembered that the occurrence of 

 Mendelian phenomena in heredity depends en- 

 tirely upon a much more fundamental phenomenon 

 —that of sex^ — which gives the experimenter the 

 'pportunity of crossing two individuals differing as 

 to on§ or more separately heritable characters, 

 and of observing the numerical proportions of the 

 offspring in which each of these characters makes 

 its appearance. 



The phenomenon of sex, as we all know, is a 

 \cry great mystery, and introduces endless com- 

 plications into life. Sexual differentiation appears 

 to be nearly as old as the cell itself. The stream 

 of life, almost since it first began to flow, has 

 been a double stream, or, better, a network, in 

 which male and female streamlets unite at more 

 or less frequent intervals to farm those temporary 

 overflows which we call individuals. Each stream- 

 let goes its own way for a time, and then joins 

 and exchanges experiences, so to speak, with 

 another. It is just this exchange of experiences 

 that forms the basis of the Mendelian phenomena, 

 and it is not merely the experiences of a single 

 lifetime, but those of many generations that may 

 be thus exchanged. 



Perhaps, however, we are getting a little too 

 metaphorical and had better consider in a rather 

 more matter-of-fact manner what actuallv takes 

 place in the sexual process. The essential feature 

 of this process is always the same — the union of 

 two germ-cells to form a single cell, although this 

 fundamental act is greatly obscured in the higher 

 plants and animals by the endless contrivances 

 which have arisen in the course of evolution, and 

 which serve the ultimate purpose of bringing the 

 germ-cells together. In all the higher animals 



NO. 2707. VOL. 108] 



and plants these germ-cells are sharply differen- 

 tiated into male and female, spermatozoa or sperm- 

 cells and ova or egg-cells, and, with rare excep- 

 tions, the egg-cell cannot even begin to develop 

 until it has united with, or, as we say, been fer- 

 tilised by, a sperm-cell. This is very literally the 

 union of two branches of the stream of life. 



From the point of view of the theory of heredity, 

 the most important thing about this union is the 

 coming together of two sets of chromosomes — 

 paternal and maternal — the one set coming with 

 the spermatozoon from the male parent, and the 

 other with the ovum from the female parent. The 

 maternal and paternal chromosomes bring with 

 them factors that have arisen in some unknown 

 way, probablv by chemical changes, in the two 

 ancestral streams of protoplasm which unite in 

 the fertilised e^^%. Apart altogether from the 

 much-vexed question of the inheritance of 

 " acquired " characters, which we cannot even 

 touch upon this evening, these factors represent 

 certain experiences which the stream of life has 

 gathered on its journey. 



Hence the new organism may exhibit certain 

 characters derived from the father and others 

 derived from the mother, a combination of paternal 

 and maternal peculiarities, while the fundamental 

 features of its organisation cannot be said to be 

 derived from one parent more than from the 

 other. It will resemble either parent just in so 

 far as it starts life with the same potentialities, 

 inherent in the germ-plasm as a whole and in its 

 special factors, and just in so far as it develops 

 under identical conditions. (We must not forget, 

 though the point is not essential to our argument, 

 that the germ-cells may perhaps contain other 

 special factors besides those which have been 

 located in the chromosomes.) 



It is a curious fact, and one upon which our 

 social reformers and preachers of equality would 

 do well to reflect more seriously, that the char- 

 acters, the potentialities for good or evil, of a 

 living being should depend so much upon mere 

 chance. A great deal can be done for the welfare 

 of the individual by improving the conditions 

 under which it lives, as every gardener knows, but 

 nothing can altogether counteract the effects of 

 hereditary tendencies. It is worth while to con- 

 sider a little more fully how it comes about that 

 chance plays such an important part. 



With certain exceptions, which do not affect the 

 general proposition, every cell of the living 

 organism contains, as we have already seen, a 

 double set of chromosomes, one set derived from 

 the male and the other from the female parent. 

 The duplication takes place at the time when the 

 two germ-cells come together to form the fer- 

 tilised ^^^y but it is counteracted again bv re- 

 duction at another period of the life-cycle ; other- 

 wise the number of chromosomes in each cell 

 would continue to increase in geometrical ratio 

 from generation to generation, which is clearly 

 impossible. It is at these two critical periods that 

 chance steps in and prepares her surprises. 



