CHAPTER XXIV 



SOME CONSEQUENCES OF EVOLUTIONARY 

 RELATIONSHIP 



We have seen that one result of evolution has been the multiplication 

 of species. This has resulted chiefly from the splitting of older species, 

 and the geological record shows that this process has been operative 

 throughout geological time. We may safely assume that a common 

 ancestor could be found for any two species by going far enough back. 



The blood relationships of organisms. We reckon kinship among 

 men by the fact of common ancestry. Brothers and sisters are the closest 

 of kin, only one generation removed from their common parents ; cousins 

 are more distant relatives, having common ancestors two or more genera- 

 tions back. So it is with species; the closeness of their blood relationship 

 depends upon how far back in time their common ancestral species 

 existed. 



Of course, the kinship between even the most closely allied species is 

 very remote compared to that which exists between cousins or even 

 between all the individuals in a single species population. The common 

 ancestor of species belonging to the same genus usually lived some 

 thousands or millions of years and countless generations ago. We must 

 go further and further into the past to reach the common ancestors of 

 species belonging to different genera, to different families, and to different 

 orders. When it comes to the origins of the plant and animal phyla, we 

 can only surmise; they are older than the oldest geological records. Yet 

 the whole trend of evolution suggests that life was once one, and that 

 all creatures made of protoplasm are descended from the original living 

 blob of that substance. If this is so, then every living thing is related to 

 every other living thing by blood ties, near or remote. 



What do we mean when we speak thus of "kinship" or " blood rela- 

 tionship" existing between individuals or species? Obviously this can 

 mean nothing more or less than the presence of common genes, inherited 

 from common ancestors. Close relatives share many or most genes; more 

 distant ones have fewer genes in common. The amount of genie difference 

 between any two species should, in a rough way, be a measure of the 



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