HEREDITY AND ENVIRONMENT 199 



which influence all forms of growth. Thus even Lewis's 

 experiment of transplanting the optic vesicle, with the 

 result of a transformation of the skin above it into a 

 rudimentary lens, will probably be finally explained as the 

 evolutionary possession by the vesicle of a catalytic secre- 

 tion activated by light which alters the form and structure 

 of the epithelial cells in close contact with it. We can 

 conceive no organ-forming substance to construct the 

 heart ; but it is easy enough to regard it as a progressively 

 formed functional adaptation to stresses imposed upon it 

 during embryonic growth in which it is, to use Starling's 

 words, " a new creation." Von Nageli and Hertwig 

 pointed out with each stage in growth the internal environ- 

 mental complexity increases. But such complexity uses 

 an increasing complexity of tools, for just as mere increase 

 of numbers in a factory without new instruments does 

 not necessarily result in new differentiations among the 

 workers or different structural developments in the build- 

 ings, so in the animal organism mere increase in bulk does 

 not imply increasing complexity. The most important 

 variables in all growth, structure, and function are the 

 " tools " used, and the engines made of them, and the 

 illustrations of the phenomena of budding and mitosis 

 given in Method in Science are probably far more than 

 illustrations of the way in which organisms in a changing 

 environment acquire the tools which change function 

 and change structure and can be transmitted, just as they 

 can be lost in another environment. 



It seems, then, as if Weismann occupies the position 

 of a mathematician who works out a set of equations in 

 which a, b, x, and y obviously represent no more than 

 possible theoretic factors leading to a conclusion which 

 is afterwards found to be near the mark as soon as the 



