THEORIES OF EVOLUTION 19 



in the conditions of selection, and therefore in the direction 

 of evolution. New qualities become essential and are evolved 

 by stringent selection ; other qualities which were formerly 

 stringently selected are now merely preserved or undergo 

 deterioration. Civilized man, for example, is to-day being 

 stringently selected by certain forms of disease, and is 

 undergoing evolution against them. But his feet, his hands, 

 his heart, his lungs, his liver, have not changed apparently for 

 thousands of years. Sight, hearing, smell, teeth, etc. are said 

 to be deteriorating. They were evolved during different but 

 overlapping periods of a long-extended past. It follows that 

 though organisms are seldom in anything like perfect adapta- 

 tion to their surroundings, organs very frequently are more 

 nearly so. It further follows, if the Neo-Darwinian doctrine 

 be wholly true, that the evolution of all races proceeds during 

 any epoch on lines of comparative simplicity. 



31. All causes of death are not causes of evolution, since 

 they may be neither selective nor stringent. Thus deaths 

 by fire and water do not cause evolution in England. Thus 

 also modern weapons of war do not discriminate in battle. 



32. The Darwinian, unlike the Lamarckian doctrine, 

 attributes evolution to injurious agencies. According to it 

 the specific mean is raised by the elimination of the unfit. 

 Occasionally the two doctrines are seemingly compatible, 

 more often they are in violent opposition. The speed of 

 antelopes may, conceivably, have been evolved both by the 

 transmission of acquirements and the survival of the fittest. 

 But their powers of resisting privation and disease cannot 

 have been so evolved. Both privation and disease cause 

 deterioration in the individual, and should therefore, if 

 acquirements be transmitted, cause deterioration of the race. 

 If acquirements be not transmissible they should cause evolu- 

 tion by the survival of the fittest. Since powers of resisting 

 privation and disease have undoubtedly arisen, it is clear, in 

 this case at least, either that acquirements have not been 

 transmitted, or that the tendency to transmission has been so 

 feeble as to be easily overcome by Natural Selection. 



33. However much the transmission of acquirements 

 among multicellular forms may have been questioned, no 

 doubts seem to have arisen hitherto in medical circles as to 

 their transmission among unicellular organisms. Biologists 

 have discussed the question to some extent ; but they have 

 merely discussed it. 1 Little or no attempt has been made 



1 Weismann formerly attributed the variations of higher animals and 

 plants entirely to amphimixis to the mixture of dissimilar germ-plasms 



