246 NATURAL HISTORY. 



they may differ among themselves in structure or appear- 

 ance. 'Varieties/ therefore, are supposed to be always 

 capable of interbreeding with the type-form of the species. 

 On the other hand, if two groups of animals or plants, 

 otherwise nearly resembling each other, are found to be 

 incapable of producing fertile offspring by intercrossing, 

 then they are regarded as constituting two distinct species. 

 The physiological test, however, is wholly inapplicable 

 to extinct organisms, where the same difficulties as to the 

 distinction between species and varieties exist as among 

 living forms. It also cannot be applied without some 

 serious reservations even as regards living beings, since 

 some organisms which are otherwise clearly recognisable 

 as distinct species, are undoubtedly capable of producing 

 fertile offspring by interbreeding. This is seen in the 

 case of various plants, and in a few instances among 

 animals. 



Lamarck was profoundly impressed with the difficulty 

 of separating species from varieties, and also with the 

 very wide range of the variability shown by many animals 

 and plants. To this point he recurs again and again. 

 ' In proportion,' says he, * as we gather together the pro- 

 ductions of nature in proportion as our collections 

 become more and more extensive, in the same propor- 

 tion do we see blank after blank filled up and our lines 

 of separation effaced. We find ourselves reduced to an 

 arbitrary determination, which at one time compels us to 

 seize on the most minute varietal differences as characters 

 of what we call a species, while at another time it forces 

 us to set down those slightly differing individuals of 

 which other observers make a species, as really nothing 



