NATURE 



[SErXEMBER 22, IQIO 



;ul the flowers were greatly altered, and presented some 

 ot the modifications of the mother plant, especially the 

 ii'atistormation of stamens into petals. These experi- 

 ments are still in progress, and it would perhaps be pre- 

 mature to lay too much stress upon them if it were not 

 lor the fact that they are so completely confirmatory of 

 me results obtained by similar methods in the animal 

 Uingdom. 



1 submit to you that evidence is forthcoming that 

 external conditions may give rise to inheritable alterations 

 ul structure. Not, however, as was once supposed, by 

 producing specific changes in the parental soma, which 

 rnanges were reflected, so to speak, upon the germ-cells. 

 1 he new evidence confirms the distinctions drawn by 

 Weismann between somatic and germinal variations. It 

 shows that the former are not inherited, while the latter 

 are ; but it indicates that the germ may be caused to vary 

 by the action of external conditions in such a manner as 

 to produce specific changes in the progeny resulting from 

 it. It is no more possible at the present time to connect 

 rationally the action of external conditions on the germ- 

 ii'lls with the specific results produced in the progeny 

 than it is possible to connect cause with effect in the 

 experiments of Herbst and Stockaid ; but when we com- 

 pare these two kinds of experiments, we are no longer 

 able to argue that it is inconceivable that such and such 

 conditions acting on the germ-plasm can produce such and 

 such effects in the next generation of adults. We must 

 accept the evidence that things which appeared inconceiv- 

 able do in fact happen, and in accepting this we remove 

 a great obstacle from the path of our intjuiries, and 

 gain a distinct step in our attempts to discover the laws 

 which determine the production of organic form and 

 structure. 



But such experiments as those which I have mentioned 

 only deal with one aspect of the problem. They tell us 

 about external conditions and the effects that they are 

 observed to produce upon the organism. They give us no 

 definite information about the internal changes which, 

 taken together, constitute the response of the organism 

 to external stimuli.' As Darwin wrote, there are two 

 factors to be taken into account — the nature of the con- 

 ditions and~lhe nature of the organism, and the latter is 

 much the more important of the two. More important 

 because the reactions of animals and plants are mani- 

 fold ; but, on the whole, the changes in the conditions are 

 few and small in amount. Morphology has not succeeded 

 in giving us any positive knowledge of the nature of the 

 organism ; and in this matter we must turn for guidance 

 to the physiologists, and ask of them how far recent 

 researches have resulted in the discovery of factors com- 

 petent to account for change of structure. Perhaps the 

 first step in this inquiry is to ask whether there is any 

 evidence of internal chemical changes analogous in their 

 operation to the external physical and chemical changes 

 which we have been dealing with. 



There is a great deal of evidence, but it is extremely 

 difticult to bring it to a focus and to show its relevancy 

 to the particular problems that perplex the zoologist. 

 Moreover, the evidence is of so many different kinds, and 

 each kind is so technical and complex, that it would be 

 absurd to attempt to deal with it at the end of an address 

 that has already been drawn out to sufficient length. 

 But perhaps I may be allowed to allude to one or two 

 generalisations which appear to me to be most suggestive. 



We shall all agree that, at the bottoir, production and 

 change of form is due to increase or diminution of the 

 activities of groups of cells, and we are aware that in the 

 higher animals change of structure is not altogether a 

 local affair, but carries with it certain consequences in 

 the nature of correlated changes in other parts of the 

 body. If we are to make any progress in the study of 

 ntorphogeny, we ought to have as exact ideas as possible 

 as to what we mean when we speak of the activities of 

 cells and of correlation. On these subjects physiology 

 supplies us with ideas much more exact than those derived 

 from morphology. 



It is, perhaps, too sweeping a generalisation to assert 

 that the life of any given animal is the expression of the 

 sum of the activities of the enzvmes contained in it, but 

 it seems well established that the activities of cells are, 



NO. 2134, VOL. 84] 



if not wholly, at all events largely, the result of the 

 actions of the various kinds of enzymes held in combina- 

 tion by their living protoplasm. These enzymes are highly 

 susceptible to the influence of physical and chemical media, 

 and it is because of this susceptibility that the organism 

 responds to changes in the environment, as is clearly illus- 

 trated in a particular case by Tower's experiments on 

 the production of colour changes in potato-beetles. 

 Bayliss and .Starling have shown that in lower animals, 

 protozoa and sponges, in which no nervous system has 

 been developed, the response of the organism to the 

 environment is effected by purely chemical means. In 

 protozoa, because of their small size, the question of 

 coadaptation of function hardly comes into question ; but 

 in sponges, many of which are of large size, the mechanism 

 of coadaptation must also be almost exclusively chemical. 

 Thus we learn that the simplest and, by inference, the 

 phyletically oldest mechanism of reaction and coordination 

 is a chemical mechanism. In higher animals the necessity 

 for rapid reaction to external and internal stimuli has led 

 to the development of a central and peripheral nervous 

 system, and as we ascend the scale of organisation this 

 assumes a greater and greater importance as a co- 

 ordinating bond between the various organs and tissues of 

 the body. But the more primitive chemical bond persists, 

 and is scarcely diminished in importance, but only over- 

 shadowed, by the more easily recognisable reactions due 

 to the working of the nervous system. In higher animals 

 we may recognise special chemical means whereby 

 chemical coadaptations are established and maintained at 

 a normal level or in certain circumstances altered. These 

 are the internal secretions produced by sundry organs, 

 whether by typical secretory glands (in which case the 

 internal secretion is something additional and different 

 from the e.xternal secretion), or by the so-called ductless 

 glands, such as the thyroid, the thymus, the adrenal 

 bodies, or by organs which cannot strictly be called glands, 

 namely, the ovaries and testes. All these produce chemical 

 substances which, passing into the blood or lymph, are 

 distributed through the system, and have the peculiar 

 property of regulating or exciting the specific functions of 

 other organs. Not, however, of all the organs, for the 

 different internal secretions are more or less limited and 

 local in their effects, one affecting the activity of this 

 and another the activity of that kind of tissue or organ. 

 Starling proposed the name hormones for the internal 

 secretions because of their excitatory properties (6p/ia&, to 

 stir up, to excite). 



Hormones have been studied chiefly from the point of 

 view of their stimulating effect on the metabolism of 

 various organs. From the morphologist's point of view, 

 interest chiefly attaches to the possibility of their regu- 

 lating and promoting the production of form. It might 

 be expected that they should be efficient agents in regu- 

 lating form, for, if changes in structure are the result of 

 the activities of groups of cells, and the activities of cells 

 are the results of the activities of the enzymes which they 

 contain, and if the activities of the enzymes are regulated 

 bv the hormones, it follows that the last-named must be 

 the ultimate agents in the production of form. It is 

 difficult to obtain distinct evidence of this agency, but in 

 some cases, at least, the evidence is sufficiently clear. 

 I will confine myself to the effects of the hormones pro- 

 duced by the testes and ovaries. These have been proved 

 to be intimately (-onnected with the development of 

 secondary sexual characters, such, for instance, as the 

 characteristic shape and size of the horns of the bull ; the 

 comb, wattles, spurs, plumage colour, and spurs in 

 poultry ; the swelling on the index finger of the male frog ; 

 the shape and size of the abdominal segments of crabs. 

 These are essentially morphological characters, the results 

 of increased local activity of cell-growth and differentiation. 

 .•\s they are attributable to the stimulating effect of the 

 •hormone produced by the male organ in each species, 

 they afford at least one good instance of the production 

 of a specific change of form as the result of an internal 

 chemical stimulus. We get here a hint as to the nature 

 of the chemical mechanism which excites and correlates 

 form and function in higher organisms, and. from what 

 has just been said, we perceive that this is the most 

 primitive of all the animal mechanisms. I submit that 



