112 EVOLUTION AND ANIMAL LIFE 



coming about specifically from the varying effects of use or 

 disuse of parts, and the functional stimulation of other parts 

 in response to such extrinsic conditions as light, contact, tem- 

 perature, pressure, color, etc., etc. The changes effected will, 

 in the nature of things, be essentially adaptive. Now, these 

 adaptive changes, these variations, or new characters acquired 

 during the lifetime of the individual will be, in Lamarck's belief, 

 inherited, if not in full, at least in partial degree, by the offspring. 

 These in turn submitted to similar or to different environ- 

 mental influences will continue the changes either cumulatively 

 or diversely. By this steady direct change and adaptation to 

 environment the species is ever modifying and transforming. 

 Evolution marches, and marches adaptively and advanta- 

 geously. 



But modern naturalists find a most unfortunate impediment 

 to this simple, direct, and sufficient explanation of species- 

 forming and evolution in the apparent untruth of the assumption 

 that the characters acquired by an individual in its lifetime are 

 transmitted by inheritance to its young. This question, fun- 

 damental to the Lamarckian theory, of the inheritance or non- 

 inheritance of acquired characters has long been one of the most 

 hotly debated points in evolution biology. As we have devoted 

 a number of pages to its particular discussion in our later chap- 

 ter on heredity (Chapter X), we need not anticipate that 

 discussion here. It is sufficient to say that as far as scientific 

 proof, that is, evidence from actual observation and experiment, 

 goes, those naturalists led by Weismann, who deny this inheri- 

 tance, have at present distinctly the better of the argument. 



The orthogenetic evolution theories of various authors, 

 based upon the assumed occurrence of variations in determinate 

 lines or directions (a restricted and determinate variation as 

 compared with the nearly infinite, fortuitous, and indeterminate 

 variation assumed in the selection theory) , are of several types. 

 The mention of tw r o will reveal pretty well the more important 

 characters of all. Not a few biologists have always believed in 

 the existence of a sort of mystic, special vitalistic force or prin- 

 ciple by virtue of which determination and general progress of 

 evolution is chiefly fixed. Such a capacity, inherent in living 

 matter, seems to include at once possibility of specific adapta- 

 tion and the possibility of progressive or truly evolutionary 

 change. Not all evolution is in a single direct line, to be 



