652 INVERTEBRATA CHAP. 



make a crab out of a lobster-like ancestor, and the whole of the 

 Tertiary Period to convert a five-toed mammal into a horse. 1 



The dislike with which the Lamarckian theory is viewed is not 

 entirely due to the supposed evidence against it furnished by experi- 

 ments. Two other considerations have been urged : first, that there 

 are modifications which cannot have been the result of the inheritance 

 of acquired characters ; and, second, that it is difficult to conceive of 

 any mechanism by which the characters of the body can be transferred 

 to the germ. 



In the first objection there lurks some confusion of thought. It 

 is tacitly assumed that acquired characters must be changes of 

 structure due to new habits acquired by the animal in adaptation to 

 its new environment. But to assume this is to narrow, in an 

 unwarrantable fashion, our conception of what constitutes an acquired 

 character. The reaction to the environment need not be an adapta- 

 tion. Certainly it will only be preserved if it happens to be an 

 adaptation, or if it is at least not harmful. But even with this 

 widening we must admit the justice of Lankester's criticism of such 

 explanations, as applied to the habits of insects in feigning death : 

 " An insect either postures and escapes, or it does not posture and it 

 is eaten ; it is not half-eaten and left to benefit from experience." 



But our aim is not to explain the origin of all specific differences 

 it is the much more modest one of attempting to give a rational 

 account of recapitulation.. 



As to the second consideration which is urged against the inherit- 

 ance of acquired characters, viz. the difference of forming a conception 

 of its modus operandi ; its great weight must be admitted, but even 

 here we are beginning to receive light from both physiology and the 

 new science of experimental embryology. The discovery of hormones, 

 by Starling, is of far-reaching importance in this respect. These are 

 substances, produced by certain organs of the body and poured into 

 the body-fluids, which have a powerful effect on growth and other 

 metabolic processes. Now, it is quite possible and even probable 

 that the few hormones wliich so far have been discovered, such as the 

 hormones of the thyroid and pituitary glands, have been recognized 

 on account of their exceptionally active chemical character, and that 

 hormones of lesser strength are given off by all the tissues of the 

 body, and that the various organs of the body stand in a sort of mobile 

 chemical equilibrium with one another. 



Considerable support can be drawn from experimental embryology 

 in support of this view. Herbst (1899) has shown that if the eye of 



1 Since these lines were written an interesting example of the stereotyping of a 

 functional reaction has occurred to us. In the lagoons of Prince Edward Island, 

 where the oyster swarms, two varieties occur : one, the so-called Cup-oyster, on hard 

 shelly or gravelly ground ; the other, the Mud-oyster, on mud. In the latter the 

 edges of both valves are curved up so as to keep the opening above the mud ; in the 

 other the edges of the valves remain horizontal. Both varieties develop from the 

 same larvae. Now in secondary strata there occurs a genus, Gryphaea, which is just 

 a Mud-oyster stereotyped, and it is found in clay deposits. 



