CHAPTER XXA'. 



BIOGENETIC RELATIONS OF PLANTS AND ANIMALS.* 



The Conception of Species. 



A species Is commonly held to comprise a group of individuals 

 which differ from one another only in a minor degree. The degree 

 of individual difference admissible within the species is commonly 

 a matter of personal opinion and probably no two systematists al- 

 ways agree as to the precise taxonomic value of a character in dif- 

 ferent cases. In pre-modern days the idea of permanence and im- 

 mutability of the species, or, in pre-Linnsean days, of the genus, 

 dominated the minds of naturalists generally, though there were 

 not wanting, at nearly all times, observers to whom the fixity of 

 specific characters appeared as a dogma unsupported by facts. 

 That variation existed within the specific limits was admitted, but 

 the believers in the special creation and immutability of species 

 would not aidmit that this variation could exceed certain limits, 

 though what these limits were was a matter of diverse and, more- 

 over, of constantly changing opinion. No matter how different 

 the end members of a perfectly graded series of individuals were, 

 if that gradation was established all those members were placed 

 within the limits of the species. Even if some of the members of 

 the series were originally described as distinct species or placed 

 in distinct genera the discovery of intermediate forms, or "con- 

 necting links," caused them all to be referred to one species. The 

 differences originally deemed amply sufficient for specific or even 

 generic distinction at once dwindled in taxonomic value to the rank 

 of varietal characters of a very variable species. A classic case 

 in point is that of the Tertiary species of Paludina (Vivipara) from 

 Slavonia. (Neumayr and Paul-29). (Fig. 252.) In the lowest 

 members of the Paludina beds P. neumayri (Fig. a), a smooth, 



* The principles here outlined will be more fully discussed in "The Principles 

 of Paleontology " by Henry F. Osborn and Amadeus W. Grabau, to be published 

 shortly. 



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