286 



The Journal of Heredity 



jects and methods of taxonomy are 

 entirely different from those of evolu- 

 tionary and genetic study. Though 

 taxonomists disregard individual diver- 

 sities and seek for the most constant or 

 general characters, the difficulty of 

 finding definitely diagnostic differ- 

 ences is well known, and is a testimony 

 that diversity is a normal and universal 

 condition of species. 



DIVERSITY UNIVERSALLY FOUND 



Close and careful observation of any 

 natural species shows not merely 

 infinitesimal diversity, but appreciable 

 heritable differences. The art of 

 breeding improved varieties is based on 

 the recognition of differences. Skill 

 and practice may be necessary to 

 detect differences readily in unfamiliar 

 species, but persistent search is always 

 rewarded. The diversity of the human 

 species is our most familiar example. 

 Other species appear more uniform 

 because less familiar, but shepherds 

 know their individual sheep, and 

 garden experts see individual differ- 

 ences in plants. Many travellers have 

 noted their first impression that native 

 Africans, Malays, or Chinese, were all 

 alike, and their later surprise in finding 

 the same people as different individ- 

 ually as Europeans. The framing of 

 definite race characters is as difficult 

 as writing satisfactory diagnoses of 

 species. 



An ideal taxonomic character is one 

 that is shared by all the members of a 

 species, and is not shared by any other 

 species. Such a character must have 

 arisen in the species and become estab- 

 lished through the whole network of 

 descent. On the other hand, large 

 numbers of hereditary differences exist 

 as forms of diversity, without becoming 

 standardized or established as uniform 

 diagnostic characters of species, but 

 continuing to appear as parallel varia- 

 tions in many related species, or even 

 in distinct genera. Thus many species 

 or genera may respond in the same way 

 to natural selection as a standardizing 

 agency, if any change of the environ- 

 ment gives a more definite survival 



value to a particular character or 

 combination of characters already 

 represented in the networks of descent. 

 Cases of parallel development may be 

 taken to prove that evolution is me- 

 chanically directed, but less confidence 

 is placed in the theory of orthogenesis 

 when account is taken of the frequency 

 of parallel variations. 



THE EXISTENCE OF UNIFORM SPECIES 

 UNPROVED 



To "prove" that no species is uni- 

 form, or "pure," is beyond the logical 

 range, like other universal negatives. 

 Though diversities have been found in 

 thousands of species, there are thou- 

 sands more that have not been in- 

 spected for diversity or bred artificially 

 to see whether they are heterozygous 

 or not. The custom of many writers is 

 to treat differences as variations and 

 assume environmental causes. Or gen- 

 etic diversity may be recognized but 

 confused with hybridism. Tendencies 

 to vary may be admitted, but uniform- 

 ity claimed as a result of natural 

 selection. According to a recent paper 

 by Professor Osborn, "... Nature 

 is constantly standardizing her ma- 

 chines through individual competition 

 and producing flocks of birds and 

 shoals of fishes which are so precisely 

 alike that animals of the same age, sex, 

 environment and heredity show no 

 perceptible variatioft ..." Thus the 

 pure species assumption is carefully pre- 

 served, notwithstanding the confusion 

 that it brings into general evolutionary 

 ideas. Selection would need to be 

 effective in rejecting all forms of diver- 

 sity, if it were to keep the members of 

 species "precisely alike." But why 

 should we invoke natural selection or 

 any other agency to explain a condition 

 of uniformity that probably does not 

 exist, and certainly has not been 

 demonstrated? 



Evolution no doubt is controlled by 

 natural selection, to the extent that the 

 adaptive characters may be favored by 

 restricting the non-adaptive, but the 

 special Darwinian doctrine of natural 

 selection as the cause and explanation 



