Organic Evolution. 215 



all the anterior part of the body should be appropriately 

 modified. 



These considerations, perhaps, somewhat weaken the 

 force of Mr. Spencer's argument, which is not quite so 

 strong now as it was when the "Principles of Biology" 

 was published. 



(2) We may pass now to the evidence afforded by direct 

 observation and experiment. There is little enough of it. 

 The best results are, perhaps, those which have been 

 incidentally reached in the poultry -yard and on the farm 

 in the breeding of domesticated animals. We have seen 

 that, under these circumstances, certain parts or organs 

 have very markedly diminished in size and efficiency ; 

 others have as markedly increased. Of the former, or 

 decrease in size and efficiency,' the imbecile ducks with 

 greatly diminished brains have been already mentioned. 

 Mr. Herbert Spencer draws attention * to the diminished 

 efficiency in ear-muscles, giving rise to the drooping ears 

 of many domesticated animals. " Cats in China, horses in 

 parts of Eussia, sheep in Italy and elsewhere, the guinea- 

 pig formerly in Germany, goats and cattle in India, rabbits, 

 pigs, and dogs in all long-civilized countries, have dependent 

 ears." t Since many of these animals are habitually well 

 fed, the principle of economy of growth seems excluded. 

 Indeed, the ears are often unusually large; it is only 

 their motor muscles that have dwindled either relatively 

 or absolutely. If what has been urged above be valid, 

 panmixia cannot have been operative ; since panmixia fer 

 se only brings about regression to mediocrity. If the effects 

 in these two cases, ducks' brains and dogs' ears, be not due 

 to disuse, we know not at present to what they are due. 

 In the correlative case of increase by use, we find it exceed- 

 ingly difficult to exclude the disturbing effects of artificial 

 selection. The large and distended udders of cows, the 

 enhanced egg-laying powers of hens, the fleetness or 

 strength of different breeds of horses, — all of these have been 



* Nature^ \ol. xli. p. 511. 



t "Animals and Plants under Domestication," vol. ii. p. 291. 



