SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 355 



time that has elapsed since their ancestors were part of a single species 

 population. 



Viewed in this light, the existence of close and remote blood ties be- 

 tween species affords a genetic explanation for the homologous resem- 

 blances that they show. It accounts for the graded degrees of such 

 resemblance, by means of which we can classify plants and animals in 

 an orderly system of ranked categories. It explains the characteristic 

 patterns of spacial distribution exhibited by groups of similar species. 

 Lastly, it furnishes a rational basis for our use of homology, taxonomy, 

 and biogeography for determining degrees of kinship among organisms. 



HOMOLOGY AS AN EXPRESSION OF RELATIONSHIP 



Determination of degree of relationship must be based upon detailed 

 and critical comparisons of organisms with respect to their similarities 

 and differences. The easiest comparisons to make are those of form and 

 structure (comparative morphology). But we may apply the same meth- 

 ods to the study of development (comparative embryology) or of function 

 (comparative physiology). Wherever we make such comparisons, we find 

 a multitude of resemblances between species. However, we soon learn 

 that the resemblances are of two different sorts, which must be clearly 

 distinguished. Some are fundamental similarities which we assume are 

 caused by the inheritance of a common stock of genes ; these are homologies. 

 Others are superficial similarities between only distantly related species, 

 resulting from independent adaptation to similar environmental require- 

 ments and produced by wholly different gene complements. These are 

 analogies and do not indicate relationship between their possessors. It is 

 easy to distinguish these two kinds of resemblance in the abstract but 

 sometimes not so easy in a specific instance. Let us look at some clear-cut 

 examples of analogy and of homology ; later we shall see how both may be 

 involved in other cases of resemblance. Here we shall consider only form 

 and structure. 



Analogy. Structures that look alike and have similar functions but 

 differ in fundamental plan and in embryological origin are said to be 

 analogous, or to show analogous resemblance. Such analogous structures 

 often occur in organisms that no one could mistake for close relatives. 

 Thus both moles (mammals) and mole-crickets (insects) have digging 

 front feet that are strikingly similar in form and function, but that differ 

 fundamentally in plan and in embryonic origin. The foot of the mole is 

 built like the hand of a man; its digging tools are the strong, flattened 

 claws at the ends of the digits. The foot of the mole cricket is an insect 

 appendage with characteristic exoskeleton and internal muscles; its 

 digging prongs are spinous projections of the body armor. The similarity 



