t62 Animal Life and Intelligence, 



satisfactory answer can be given to the question — How can 

 the body affect the germ so that this or that particular 

 modification of body-cells may be transmitted to the 

 offspring? We may make plausible guesses, or we may 

 say — I know not how the transmission is effected ; but there 

 is the indubitable fact. 



This leads us to the evidence of the fact. 



It must be remembered that no one questions the 

 modifiability of the individual. That the epidermis of the 

 oarsman's hand is thickened and hardened ; that muscles 

 increase by exercise; that the capacity for thinking may 

 be developed by steady application ; — these facts nobody 

 doubts. That well-fed fish grow to a larger size than their 

 ill-fed brethren; that if the larger shin-bone (the tibia) 

 of a dog be removed, the smaller shin-bone (the fibula) soon 

 acquires a size equal to or greater than that of the normal 

 tibia ; that if the humerus, or arm-bone, be shifted through 

 accident, a new or false joint will be formed, while the old 

 cavity in which the head of the bone normally works, fills 

 up and disappears ; that canaries fed on cayenne pepper 

 have the colour of the plumage deepened, and bullfinches 

 fed on hemp-seed become black ; that the common green 

 Amazonian parrot, if fed with the fat of siluroid fishes, 

 becomes beautifully variegated with red and yellow ; that 

 climate affects the hairiness of mammals ; — these and many 

 other reactions of the individual organis m in response to 

 environing conditions, will be admitted by every one.* 

 That constitutional characters of germinal origin are in- 

 herited is also universally admitted. The difficulty is to 

 produce convincing evidence that what is acquired is really 

 inherited, and what is inherited has been really acquired. 



Attempts have been made to furnish such evidence by 

 showing that certain mutilations have been inherited. I 

 question whether many of these cases will withstand rigid 



* Mr. J. A. Thomson has published a most valuable " Synthetic Summary 

 of the Influence of the Environment upon the Organism " (Proceedings Koyal 

 Physiological Society, Edinburgh : vol. ix. pt. 3, 1888). The case of the 

 Amazonian parrots was communicated to Darwin by Mr. Wallace (" Animals 

 And Plants under Domestication," vol. ii. p. 269). 



