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 
