Convergence and Divergence 109 



cricket and the mole cricket may be taken to illustrate this 

 idea, since the two forms are sufficiently alike to be recog- 

 nized as of the same family by the casual observer, and 

 yet show marked traits that are obviously related to habits 

 of life. 



The importance of this dynamic aspect of adaptation 

 was long ago recognized in connection with problems of 

 classification. Richard Owen, a comparative anatomist who 

 extended Cuvier's work in many directions, found it neces- 

 sary to make the distinction between organs and structures 

 that were " really " the same, and those that only " seemed " 

 to be alike. For this purpose he invented the terms ho- 

 fnologous for the former and analogous for resemblances re- 

 lated to function. It was also a simpler matter to describe 

 relative positions of organs than to find out exact facts about 

 functions. There was a tendency to disregard analogies in 

 systematic descriptions and the emphasis came to be laid upon 

 the equivalence or homology of parts. Later, Ray Lankester 

 developed the distinction still further since it is not always 

 easy to apply Owen's tests. He accordingly proposed the 

 terms Homogeny and Homoplasy to denote these two con- 

 cepts: First, the similarities or identities due to covtmon ori- 

 gins; and Second, the similarities or identities due to adapta- 

 tion to comtnon junctions or living conditions. 



President Henry Fairfield Osborn has unified thousands 

 of cases from the systematists and field students into two 

 general laws: 



1. The law of adaptive convergence or parallelism of 

 form, being the production of externally similar forms in 

 adaptation to similar external natural forces. 



2. The law of adaptive radiation or divergence, being 

 the tendency of descendants of a primitive stock to develop 

 differences of form while radiating into areas of unlike living 

 conditions. 



Without being able to say just how the adaptations, 

 whether convergent or divergent, are brought about, the 

 biologist can support these general statements with abun- 



