416 TRANSACTIONS OP SECTION D, 



the change in the external organ is of the nature of a reaction to a stimulus and 

 when we contemplate the marvellous changes in growth due to minute quantities 

 of organ-forming substances, then the problem becomes altogether changed, and 

 the possibility of its solution brought nearer. The whole study of comparative 

 embryology seems to support some such conclusion as this, for we find a con- 

 stant tendency in the more specialised types of development for changes which 

 must have corresponded to changes in enviromnent to be pushed back to 

 successively earlier stages in the life-history. As Hyatt has shown, the study 

 of youth-stages of fossil Cephalopoda where the evidence is available points in 

 the same direction. Now, we can find evidence of the same thing in these 

 organ-forming stimuli. We have seen that the formation of an eye in the 

 shrimp is due to an influence emanating from the optic ganglion, and that if 

 eye and ganglion be both removed the wounded ectoderm heals up and forms 

 an antenna. But if the same experiment be performed on the more modified 

 crab a different result follows : whether the optic ganglion be removed or 

 not, a new eye is regenerated. We may regard the optic ganglion as forming, 

 as it were, a kind of internal environment for the ectoderm, and in the more 

 modified crab the influences wliich radiate from this internal environment have 

 become, so to speak, stored up in the nuclei of the ectoderm, so that these now 

 have in themselves the capacity of the formation of an eye independently of 

 any stimulus. 



Of course, by experimental embryology we can never demonstrate the facb 

 that the action of the environment ever is imprinted on the genital cells and 

 that acquired characters actually are inherited. At most we can find examples 

 of possible modus o^Krandi of this influence. The final proof must be sought 

 in breeding experiments. Before, however, we complain of the paucity of 

 results obtained from these, let us clearly grasp the difficulties of obtaining a 

 definite result at all in such a case. We may expose animals to a changed 

 environment and observe that changes in their structure result; if we obtain 

 offspring from them, and rear these in the normal environment, we shall most 

 probably find that the change in structure has been entirely lost, and therefore 

 many biologists infer that these environmental changes are not inheritable. But 

 in drawing tliis conclusion such biologists entirely forget that, if a change from 

 one environment to another causes a change in structure in one generation, a 

 change in the opposite direction should be sufficient to reverse it in an equal 

 amount of time. On the other hand, if a change in structure is orJy caused by a 

 changed environment after exposure to it through a number of generations, then, 

 when the changed offspring are retransferred to a normal environment, the 

 changed structure should persist in a diminishing degree for a number of genera- 

 tions ; but the successful carrying out of such an experiment would require a 

 long period of years, and very few such experiments have been attempted. 

 Kammerer, however, has published an account of such an experiment proving 

 the inheritability of the effects of environment in the skin colour of the 

 Salamander, which in my opinion is conclusive; and he rightly says that those 

 who would follow in his footsteps and perform similar experiments must be 

 prepared to consecrate to them a considerable portion of their lives. 



In conclusion, we may say that the labours of experimental embryologists 

 have allowed us to obtain a glimpse into the nature of the forces which trans- 

 form the apparently simple and formless germ into the complicated adult 

 animal, and, though at present we are unable to compare these forces with 

 forces which act on non-living matter, yet at any rate we are enabled to classify 

 them and to learn something about their laws of action ; and this knowledge is 

 an indispensable preliminary to any deeper knowledge of their nature to which 

 we may hope that in the future we may be able to attain. 



We have seen that Driesch's conception of an indwelling entelechy, though 

 logically defensible, is useless and unworkable in practice, and that the concep- 

 tion of the existence of organ-forming substances fits in much better with the 

 facts, although these hypothetical substances are very different in their nature 

 from the ordinary chemical substances found in inorganic nature. Finally, we 

 have seen that the growing organs of the individual constitute, so to speak, 

 an environment for one another, and many features of the adult are due to 



