CHAPTER XVIII 



ACQUIRED CHARACTERS— THE PROBLEM OF PARENTAL MODI- 

 FICATIONS 



The remarkable case of the Hoatzin and the bearing thereof on the problem 

 of parental modification. The results of feeding experiments, and changes of light 

 and temperature on Flamingoes and Tanagers. 



THAT living organisms vary, in all directions, is an in- 

 controvertible fact ; but the origin of these variations 

 is still a matter for speculation. For the most part 

 Biologists are agreed that such variations are the outward and 

 visible signs, the expression points, so to speak, of the inherent 

 instability of the germ plasm, which, like all living matter, 

 being in a state of flux, can never repeat itself in all particulars 

 as one generation succeeds another. On the other hand, there 

 are those who believe that the varied forms of living organisms 

 are not to be regarded as the products of an unstable germ 

 plasm which have passed the sieve of " natural selection," but 

 rather as the products of the somatoplasm — the adult organism 

 — beaten into shape by the complex forces of the external 

 world, animate and inanimate. That is to say, they hold 

 that living matter possesses the inherent property of mallea- 

 bility : that whatever shape may be impressed, by use, upon this 

 or that part of the body, that shape is inevitably transferred and 

 added to by succeeding generations so long as the force which 

 started the modification in question remains active. In other 

 words, each individual that is born into the world of necessity 

 adds both quantitatively and qualitatively to the characters 

 " acquired " by its parents. 



No evidence has yet been brought forward, however, which 

 can be regarded as lending any weight to this contention, of 

 the transmissibility of characters acquired during adult life, 

 as distinct from those with which they were originally endowed. 



To apply this to the case of birds. That organs are modi- 



311 



