358 THE ZOOLOGIST. 



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 bottom, 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 struc- 

 ture is not altogether a local affair, but carries with it certain conse- 

 quences in the nature of correlated changes in other parts of the 

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

 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 enzymes contained in it, but it seems well established that the 

 activities of cells are, if not wholly, at all events largely, the result 

 of the actions of the various kinds of enzymes held in combination 

 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 illustrated 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 phjdetically oldest 

 mechanism of reaction and co-ordination 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 peri- 

 pheral 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 overshadowed 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 

 under certain circumstances altered. These are the internal secre- 

 tions produced by sundry organs, whether by typical secretory 

 glands (in which case the internal secretion is something additional 

 and different from the external 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 



