The Theory of Natural Selection. 253 



not imagine that such is really the case. I make the 

 assumption for the purposes of systematic exposition, 

 and in order that different parts of the subject may be 

 kept distinct. I confess it does appear to me remark- 

 able that there should still be a doubt in any educated 

 mind touching the general fact of evolution ; while it 

 becomes to me unaccountable that such should be 

 the case with a few still living men of science, who 

 cannot be accused of being ignorant of the evidences 

 which have now been accumulated. But in whatever 

 measure we may severally have been convinced or re- 

 mained unconvinced on this matter, for the purposes 

 of exposition I must hereafter assume that we are all 

 agreed to the extent of regarding the process of 

 evolution as, at least, sufficiently probable to justify 

 enquiry touching its causes on supposition of its 

 truth. 



Now, the causes of evolution have been set forth in 

 a variety of different hypotheses, only the chief of 

 which need be mentioned here. Historically speaking 

 the first of these was that which was put forward by 

 Erasmus Darwin, Lamarck, and Herbert Spencer. 

 It consists in putting together the following facts 

 and inferences. 



We know that, in the lifetime of the individual, 

 increased use of structures leads to an increase 

 of their functional efficiency; while, on the other 

 hand, disuse leads to atrophy. The arms of a black- 

 smith, and the legs of a mountaineer, are familiar 

 illustrations of the first principle : our hospital wards 

 are full of illustrations of the second. Again, we know 

 that the characters of parents are transmitted to their 

 progeny by means of heredity. Now the hypothesis 



