14b 



THE AMERICAN NATURALIST 



[Vol. XLVI 



will take place in an orderly sequence, in consequence 

 of the fixed specific properties of the elements involved. 

 Now any existing species of plant or animal is a 

 similar union of diverse elements and the possibilities of 

 its development would seem to be limited by the same 

 conditions which limit the formation of the chemical 

 compound, namely, the nature of the elements and 

 the orderly sequence of the reactions. (In either 

 case external conditions, the environment, would make 

 a profound difference, but for simplicity's sake we may 

 omit reference to that influence.) As long as one be- 

 lieves that the elements composing a species are poten- 

 tially variable in all directions, it is evident that only the 

 pressure of external conditions can prevent an indefinite 

 and unmeaning variety in the product. Such a belief 

 results in making natural selection through the environ- 

 ment the supreme directive agent in evolutionary prog- 

 ress, and really puts more responsibility upon that im- 

 portant factor than it can reasonably be expected to 

 bear. But as soon as it is shown that the elements in- 

 volved are persistently unchanging to a remarkable de- 

 gree, it becomes clear that an orderly sequence in their 

 successive interactions will follow just as in the forma- 

 tion of a chemical compound. Now biotypes are the 

 biological elements which enter into the formation of a 

 species, and the discovery of their existence and apparent 

 persistency makes the existence of an orderly sequence 

 in development quite comprehensible and indicates 

 clearly why diversity is so rarely haphazard. As in the 

 chemical synthesis, used as an illustration, the final re- 

 action follows necessary antecedent reactions, so in the 

 development of the species the last step necessarily de- 

 pends on the preceding, and the evolution of a group is 

 therefore bound to be strictly linear and in definite direc- 

 tions. Now just as in a chemical synthesis without add- 

 ing to or subtracting from its original constituent ele- 

 ments the process may be stopped, altered or accelerated, 

 either by addition or removal of some substance or by 



