September 15, 1921] 



NATURE 



85 



lite, based upon a greater complexity of bodily 

 mechanism. 



In all that has been said hitherto, which must 

 be already very familiar to most of you, we have 

 heen endeavouring to pave the way for the con- 

 sideration of what is perhaps the most difficult and 

 certainlv the most vigorously discussed problem of 

 biologv — the problem of heredity. With regard 

 to single-celled organisms such as the amoeba, this 

 problem scarcely exists. Division of the parent 

 cell entails division of all that that cell possesses. 

 The daughter-cells resemble the mother simply 

 because they are that mother divided into two 

 equal and similar parts. 



With the higher organisms, each composed, 

 perhaps, of many millions of cells, differentiated 

 into manv different kinds, and building up the 

 most diverse tissues and organs, the situation is 

 verv different. In such a case how can a single, 

 apparently undifferentiated, germ-cell, which has 

 never taken part in the formation or in the activi- 

 ties of the body as a whole, and exhibits none 

 of the features which cTiaracterise the tissue- 

 cells — how can such a simple cell give rise by 

 growth and multiplication to all the different 

 kinds of cells, arranged in all the different tissues 

 and organs, more or less exactly as in the parent? 

 The development of such an infinitely complex 

 organism as, for example, the human body, from 

 a microscopic egg-cell of apparently simple 

 structure, seems, indeed, a kind of miracle, and 

 the more closely we compare parent and child 

 the more miraculous does the result appear, for 

 not only is there a general resemblance in all 

 essential features, but there is very frequently 

 also a particular resemblance in minute peculiari- 

 ties, such as the colour of the hair or eyes, the 

 contour of the features, and so on. 



It would be claiming far too much to say that 

 we have as yet arrived at any complete explana- 

 tion of heredity — this marvellously accurate repro- 

 duction in the child of the most minute details of 

 bodily and mental organisation exhibited by the 

 parent. But the explanation is, perhaps, after all, 

 not quite so difficult as it seems at first sight. 

 Let us go back to our loaves of bread and ask 

 ourselves why one loaf resembles another. Why 

 does the loaf that is baked on Tuesday resemble 

 that which was baked in the same oven on 

 Monday? The answer is obvious. One loaf re- 

 sembles another because it is made from the same 

 kind of dough and subjected to the same kind of 

 treatment. If you take a different kind of dough, 

 or subject the same dough to a different treatment, 

 you will get a different result — and, as every 

 housewife knows, there may be a vast difference 

 between the loaves turned out by different bakers. 

 The characters of the loaf clearly depend upon 

 tw^o sets of conditions : first, the nature of the 

 dough itself, whether, for example, it is mixed 

 with yeast or baking powder, water or milk, salt 

 or sugar, and so on ; and, secondly, the nature of 

 the treatment to which the dough is subjected, 

 the shape of the tins in which it is baked, the 



NO. 2707. VOL. I08I 



temperature of the oven, and so forth. If all the 

 conditions are accurately repeated for successive 

 batches of loaves, then the loaves of each batch 

 will resemble those of the preceding batch. 



We have in this respect a very close analogy 

 with what takes place in heredity. The ^%^ con- 

 sists of a certain quantity of germ-plasm, and this 

 germ-plasm has certain characteristic peculiarities 

 of its own. In order that it may develop into an 

 adult organism like the parent, it must be sub- 

 jected to a certain treatment. In the case of a 

 hen's ^^^ undergoing incubation, or of the human 

 foetus developing in the womb of the mother, we 

 may truthfully say that it has to be baked in an 

 oven at a particular temperature. Only if all the 

 conditions are accurately fulfilled will the ^^^ 

 develop into an organism resembling the parent, 

 and it does so simply because the same causes 

 must always produce the same effects. If you 

 start with identical germ-plasm and expose it to 

 identical conditions during its development, you 

 must get an identical result. The child must 

 resemble the parent. It is, indeed, easy to show- 

 by experiment that if you var\- the conditions you 

 may get e'ther no result at all or a different one. 

 Up to a certain point, however, the living 

 organism has the power of counteracting acci- 

 dental influences, and thereby maintaining its 

 normality of structure. In other words, it is self- 

 regulating, and seems to be always endeavouring 

 to carrj- out the plan of structure characteristic of 

 the species to which it belongs, so that, if this 

 plan be disturbed, it will, within limits, be restored 

 again by appropriate growth and readjustment. 

 This power of adhering to a predetermined 

 structural plan, in spite of disturbing influences, 

 is one of the most distinctive attributes of living 

 beings, and must on no account be lost sight of 

 in considering the problem of heredity ; but at the 

 same time it is a ix)wer that is strictly limited, 



A well-known American investigator. Prof. 

 Stockard, has recently shown, in the case of vari- 

 ous animals, how abnormalities can be produced 

 by simply lowering the temperature during 

 development. Some years ago the same observer 

 obtained even more surprising results by the use 

 of a simple chemical reagent. He exposed the 

 eggs of the .American sea-minnow (Fundulus) to 

 the action of magnesium chloride, and found that 

 the young fish tended to develop with a single 

 eye in the middle of the head, instead of one on 

 each side, though the modification was not in all 

 cases complete. Thus we see that it is f)ossible, 

 by the application of a specific chemical stimulus 

 to the ^%^s to bring about a profound and perfectly 

 definite change in the structure of the organism, 

 though we are still far from knowing why this 

 should be the case. 



We also know, from recent physiological re- 

 search, that the growth of various organs in the 

 \ animal body is normally controlled by infinitesimal 

 quantities of chemical substances secreted by the 

 ductless glands, such as the thyroid and the pitui- 

 tary, and circulated in the blood, and that any 



