Aim and Method of Comparative Anatomy 



367 



and modes of life. In such a case, a single genetic point (an individual 

 or a species) is the origin of lines of progressive modification which 

 diverge into diversity — adaptive radiation (Fig. 291). 



BARRIER 



Fig. 291. Adaptive Radiation and Parallelism. 



The central territory (white) is the primitive 

 habitat of a group of animals (A). This territory is 

 surrounded by regions which differ from it and 

 from one another in their physical and biologic 

 characteristics. The primitive group invades all of 

 these regions, undergoing structural modifications 

 appropriate to each of the different environments. 

 If A is a genus, A 6 , A c , A d , and A e may be species — 

 Adaptive Radiation. 



The region south of the central territory is 

 homogeneous in its characteristics, but it is divided 

 into eastern and western areas by a barrier which is 

 impassable to animals of the A group. The group 

 invades the two areas. The original invaders were 

 genetically similar. Encountering similar environ- 

 ments on either side of the barrier, the eastern and 

 western races (A / ) undergo similar adaptive modi- 

 fications — Parallel Evolution. 



The reverse of adaptive radiation is a possibility, and one which has 

 been realized — to the confusion, at least temporarily, of comparative 

 anatomists. From two or more points far separated from one another 

 in time, place, and genetic relationship, there may arise lines of progres- 

 sive modification which converge into similarity (Fig. 292). Striking 

 examples of such convergence are found among the aquatic verte- 

 brates. Whales, sea cows, seals, sea lions — all mammals, have more 

 or less definitely fishlike form of body and, in general, similar adapt a- 



