366 Comparative Anatomy — Its History, Aim, and Method 



toral fin "A" of an ancient fish, to become the foreleg A" of a reptile 

 and, with further modification, to become a bird's wing A xrb . 



Parallelism, Adaptive Radiation, Convergence 



Certain results of experimental genetics introduce some complica- 

 tion into the concept of homology. Reverting to Fig. 290, if the change 

 which converted "a" into ax is represented as having occurred in only 

 a single individual, all the other members of the generation retaining 

 the old organ "A," that individual would be the "common ancestor" 

 of races of animals possessing A* or various modifications of A x . There 

 is reason to believe, however, that similar modifications of an organ may 

 arise in several or many individuals of a species quite independently, at 

 different times and at geographically distant points (Fig. 291). If this 

 should happen, the "common ancestor" of animals having some partic- 

 ular modification of organ "A" would be not any individual animal 

 (or pair), but rather the species — or, collectively, the germ-plasm of 

 the species. Nevertheless, the modified "A" organs are related by 

 genetic descent, whether the "common ancestor" is to be thought of as 

 an individual animal, or as numerous individuals whose germ-plasm 

 is the similarly constituted germ-plasm of all members of the species, 

 or as merely the primitive germ-plasm itself. 



It is apparently possible, therefore, that two or more lines of descent 

 (Fig. 291, A f -A f ) originating within the same species (not from a single 

 individual or pair), but otherwise quite separate from one another in 

 time or geographically, may undergo progressive modifications which 

 are closely similar or even identical in nature. The result would be that 

 new types of animals produced along the several lines could be classified 

 as belonging to the same or closely allied species. This parallel but 

 separate evolution of similar structure, whether affecting a single organ 

 or an entire organism, is appropriately called "parallelism." (Unfor- 

 tunately that term has sometimes been confusingly applied to another 

 evolutionary process better known as "convergence" (see p. 367).) 



Figure 290 illustrates the genetic modification of only the one 

 organ, "A." As a matter of fact, change in one organ is usually asso- 

 ciated with changes in others. It may be imagined, then, that the 

 individual shown on the right in Generation II of the diagram is the 

 point of departure of lines of descent which come to differ from one 

 another in that the variations or mutations which occur from time to 

 time in several or most of the organs are not alike in all the lines. It 

 may be imagined further that unfavorable changes are weeded out by 

 "natural selection," with the result that the progeny differentiate into 

 several new species, or larger groups, adapted to various environments 



