EVOLUTION AND NATURAL SELECTION 17 



heredity, ontogeny, environment, and selection; these have been 

 inseparable and interacting from the beginning; a change 

 introduced or initiated through any one of these factors causes 

 a change in all. First, that while inseparable from all the 

 others, each process may in certain conditions become an initia- 

 tive or leading factor; second, that in complex organisms one 

 factor may at the same time be initiative to another group of 

 characters, the inseparable action bringing about a continuously 

 harmonious result." 



Variation 



Since Darwin's "Origin of Species" and especially in the 

 last decade since DeVries' " Mutationstheorie " appeared, the 

 statistical, quantitative, and experimental study of variation 

 has received a new impetus, and there has developed a 

 "biometric school." Experimental investigation of variation 

 has made more exacting the study of the problem, but as yet 

 no fundamental laws have been proved regarding the real 

 causes of variation. As soon as a hypothesis is advanced, 

 it is subjected to painstaking test and analysis, principally 

 by breeding experiments of plants and animals. Variations 

 dependent on environment or use are not now supposed to be 

 inherited, while on the other hand, new characters are trans- 

 mitted from parent to offspring. On this basis a classification 

 of variations is formed on their heritability. 



According to Jordan and Kellogg ^ variations may be either 

 congenital or acquired; that is, may be such as are apparently 

 determined in the organism at conception, or such as are 

 imposed on it during its development by the influence of 

 extrinsic factors. Or variations may be divided into deter- 

 minate and indeterminate; that is, those (if there really 

 are such) which are apparently controlled by some, to us 

 unknown, influences and by these influences confined to 

 certain lines or directions of change; and, on the other 

 hand, those which are apparently wholly accidental, or 

 rather which may represent any conceivably possible line 

 or kind of change. Finally, variations may be distinguished 

 as to their general character as continuous and discon- 

 ' "Evolution and Animal Life," pp. 140, 141. 



