6 ADDRESS TO THE BH1TTSH ASSOCIATION, 1881. 



and disuse of organs, sexual selection, &c. had to be 

 taken into consideration. Passing on to the difficulties 

 of his theory, he accounted for the absence of inter- 

 mediate varieties between species, to a great extent, by 

 the imperfection of the geological record. Here, how- 

 ever, I must observe that, as I have elsewhere remarked, 

 those who rely on the absence of links between different 

 species really argue in a vicious circle, because wherever 

 such links do exist they regard the whole chain as a 

 single species. The dog and jackal, for instance, are 

 now regarded as two species, but if a series of links 

 were discovered between them they would be united 

 into one. Hence in this sense there never can be links 

 between any two species, because as soon as the links 

 are discovered the species are united. Every variable 

 species consists, in fact, of a number of closely connected 

 links. 



But if the geological record be imperfect, it is still 

 very instructive. The further palaeontology has pro- 

 gressed, the more it has tended to fill up the gaps 

 between existing groups and species : while the careful 

 study of living forms has brought into prominence the 

 variations dependent on food, climate, habitat, and other 

 conditions, and shown that many species long supposed 

 to be absolutely distinct are so closely linked together 

 by intermediate forms that it is difficult to draw a satis- 

 factory line between them. Thus the European and 

 American bisons are connected by the Bison priscus of 

 Prehistoric Europe ; the grizzly bear and the brown 

 bear, as Busk has shown, are apparently the modern 

 representatives of the cave bear ; Flower has pointed 

 out the palasontological evidence of gradual modification 



