Robertson : Theories of Evolution. 21 
worked by steam, and that the sails are comparatively useless 
—imagine, that is, that he proves to be wrong in his second 
theory, which he propounded to answer the question, ‘ ow does 
the ship move?’ This error does not in the least shake the 
truth of his first theory, namely, that the ship ts moving tn a 
certain direction. The question of how this is brought about is 
secondary, and has nothing whatever to do with the primary 
question as to whether movement in a certain direction is or is 
not occurring. 
We will now shortly consider Darwin’s Natural Selection 
hypothesis. There is much evidence that since man first began 
to domesticate animals and plants these have changed enormously 
under his hands. For instance, there are now more than a score 
of distinct breeds of pigeons, which an ornithologist, if he met 
them in the wild state, would probably name as distinct species, 
but which, according to the common opinion of naturalists, are 
all descended from the common rock pigeon. According to 
Darwin, who made a special study of these birds, all the 
different races have been produced by ‘artificial selection’ 
carried on through many generations. The expression ‘artifical 
selection’ almost explains itself, but it may perhaps be well to 
say a few words about it. No two individuals belonging to 
a species are exactly alike, and domesticated animals are 
particularly apt to vary. A man possessing domesticated 
animals or plants showing considerable variation among them- 
selves will naturally choose those to breed from in each 
generation which possess in the highest degree the characters 
which he particularly wants; that is, he will pick out the most 
favourable variations. For instance, if he is breeding race- 
horses, he will select his fleetest animals in the expectation 
that their offspring will resemble them. Or if he has a fancy 
for pigeons with exaggerated tails, he will always by preference 
breed from the individuals with the longest tail feathers. The 
idea of improving domesticated animals and plants by selection 
is by no means a modern one, though it has only been reduced 
to a definite method in comparatively recent times. The effect 
of artificial selection in producing differing races of domesticated 
animals and plants is quite extraordinary. This is brought 
home to us when we think of the difference between the race- 
horse and the dray-horse, the greyhound and the bloodhound, 
the crab-apple and Cox’s orange pippin, and realise that it is 
more than probable that these differences have been produced 
by selecting and breeding from those individuals in each 
1907 June I. 
