120 DARWINISM chap. 



and recognition by members of the same variety or incipient 

 species. It has also been observed that each differently 

 coloured variety of ■wild animals, or of domesticated animals 

 which have run wild, keep together, and refuse to pair viith 

 individuals of the other colours ; and this must of itself act to 

 keep the races separate as completely as 2:»hysical isolation. 



On the Advance of Organisation hy Natural Selection. 



As natural selection acts solely by the preservation of use- 

 ful variations, or those which are beneficial to the organism 

 under the conditions to which it is exposed, the result must 

 necessarily be that each species or group tends to become more 

 and more improved in relation to its conditions. Hence we 

 should expect that the larger groups in each class of animals 

 and plants — those Avhich haA'e persisted and have been abundant 

 throughout geological ages — would, almost necessarily, have 

 arrived at a high degree of organisation, both physical and 

 mental. Illustrations of this are to be seen everywhere. 

 Among mammalia we have the carnivora, which from Eocene 

 times have been becoming more and more specialised, till they 

 have culminated in the cat and dog tribes, Avhich have reached 

 a degree of jDerfection both in structure and intelligence fully 

 equal to that of any other animals. In another line of 

 development, the herbivora have been specialised for li\dng 

 solely on vegetable food till they have culminated in the sheep, 

 the cattle, the deer, and the antelopes. The horse tribe, 

 commencing Avith an early four-toed ancestor in the Eocene 

 age, has increased in size and in perfect adaptation of feet and 

 teeth to a life on open plains, and has reached its highest per- 

 fection in the horse, the ass, and the zebra. In birds, also, Ave 

 see an adA^ance from the imperfect tooth-billed and reptile- 

 tailed birds of the secondary epoch, to the AA'onderfully 

 dcA'eloped falcons, croAA's, and SAvalloAvs of our time. So, the 

 ferns, lycopods, conifers, and monocotyledons of the palaeozoic 

 and mesozoic rocks, have deA^eloped into the marvellous wealth 

 of forms of the higher dicotyledons that now adorn the earth. 



But this remarkable adA'ance in the higher and larger gi'oups 

 does not imply any universal laAv of progress in organisation, 

 because we haA^e at the same time numerous examples (as has 

 been already pointed out) of the persistence of lowly organised 



