548 - Heredity and Evolution 



shoots have perished; in a third species, both 

 the parent form and some of its offshoots 

 have survived; and lastly, the other species 

 of the original group have become entirely 

 extinct, leaving no descendants, after surviv- 

 ing for a shorter or a longer period. At the 

 end of 10,000 generations, the number of dis- 

 tinct forms — species or varieties — may be no 

 greater than originally, although a much 

 greater number of forms has meanwhile ap- 

 peared and disappeared. The surviving spe- 

 cies, also, are related more or less closely to 

 each other, because of their method of origin. 

 Thus the original group of species has 

 given rise to a number of more sharply sepa- 

 rated — or more distantly related — species, 

 which lend to assemble themselves, according 

 to relationship, into several larger groups. 

 Perhaps these larger groups are already suffi- 

 ciently distinct to be considered as separate 

 genera, or perhaps the separative action of 

 a greater number of generations will be re- 

 quired before this point is reached. In any 

 event, natural selection, operating through 

 ages of time, continues the fateful processes 

 of diversification and extinction. It creates 

 new species, not abruptly and de novo, but 

 gradually, from pre-existing species. And as a 

 result of the relentless processes of natu- 

 ral selection, working in conjunction with an 

 ever-changing environment, man finds him- 

 self upon this earth today — together with a 

 tremendous variety of other living species, all 

 variously adapted to current environmental 

 conditions, and all variously interrelated as 

 to their origins. Also man finds within the 

 earth itself the telltale remnants of a vastly 

 greater number of plant and animal species, 

 which have become extinct in past ages 

 (Chap. 29). 



NATURAL SELECTION AND MODERN 

 GENETICS 



The origin of species by natural selection 

 was first explained by Darwin in 1859; but 

 at that time very little was known about the 

 laws and mechanisms of hereditary and varia- 



tions. Darwin based his conclusions on the 

 unquestionable facts: that all organisms do 

 vary; that some variations are heritable; and 

 that large variations are generally less viable 

 than slighter ones. Since Darwin's time — 

 chiefly in the last 40 years — the mechanisms 

 of heredity have been thoroughly clarified 

 and much has been learned about the origin 

 and nature of variations. Prior to these de- 

 velopments, the chief objections to the theory 

 of natural selection were that the theory did 

 not account for the appearance of many ap- 

 parently useless features in different organ- 

 isms; and that variations, especially slight 

 ones, would be lost by "dilution" when the 

 variant individuals continued to interbreed 

 with the more numerous members of the 

 species not possessing the particular varia- 

 tions in question. 



Both of these objections are eliminated 

 completely by an understanding of genetic 

 principles. The visible differences between 

 closely related species are not necessarily of 

 critical importance in survival, since fre- 

 quently these visible characteristics are inci- 

 dental effects of the same genes that pro- 

 duce invisible effects having great survival 

 value. Furthermore, the idea of "dilution" 

 has proved to be altogether fallacious: when 

 mutant individuals continue to cross with 

 the original type, the processes of particulate 

 inheritance guarantee a distribution of the 

 mutant gene throughout the whole popula- 

 tion without an} 1 alteration of the new gene 

 throughout successive generations. 



Imperfections in an organism, or the pos- 

 session of features that seem to reduce its 

 fitness in relation to its particular environ- 

 ment, are not difficult to understand if the 

 limitations of natural selection are carefully 

 considered. Natural selection cannot produce 

 favorable variations, it can only preserve 

 them as, by chance, they do arise. The pos- 

 sibilities of favorable variations are strictly 

 limited by the existing genetic constitution 

 of the particular organism: the possible ways 

 in which its various genes can undergo 

 change without disrupting the delicate bal- 



