Robertson : Theories of Evolution. 213 
Survival of the Fittest. Animals and plants are subject to 
immensely severe competition, on account of the high rate at 
which they all tend to increase if they are unchecked. The 
excess of individuals produced over the number which have any 
chance of surviving is quite startling. Linnaus calculated that 
if an annual plant produced only two seeds, and its seedlings 
two each, and so on, in twenty years the descendants of the 
original plant would number one million! So it is clear that 
stringent checks to increase must be perpetually operating to 
keep the numbers of plants and animals down to those which 
we meet at the present day. Darwin dug and cleared a piece 
of ground three feet by two feet so that there could be no 
choking from other plants, and marked all the seedlings of 
native weeds that cameup. Of 357, 295 were destroyed, chiefly 
by slugs and other insects, and yet he had eliminated what is 
probably the chief cause of seedling mortality, namely, germin- 
ation in ground already stocked with other plants. Weare too 
apt to think of the struggle for existence among plants and 
animals as being a struggle against the elements only. But 
since SO many more animals and plants are produced than can 
possibly survive, the struggle is really in the first place a 
competition with other organisms, notably individuals of the 
same species, since they frequent the same districts, require the 
same food, and are exposed to the same dangers. In the 
struggle for existence Natural Selection may be compared to a 
sieve which is perpetually sifting out all but the more favourable 
varieties. ‘It may metaphorically be said that natural selection 
is daily and hourly scrutinising, throughout the world, the 
slightest variations ; rejecting those that are bad, preserving 
and adding up all that are good; silently and insensibly work- 
ing, whenever and wherever opportunity offers, at the improvement 
of each organic being in relation to its organic and inorganic 
conditions of life.’ 
As a corollary to the theory of Natural Selection, Darwin 
brought forward a subsidiary hypothesis, that of Sexual 
Selection, which had been originally suggested by his grand 
father, Erasmus Darwin. According to this theory the 
secondary sexual characters in the male arise through 
the struggle between the individuals of the male sex for 
the possession of the female. In those animals in which the 
‘law of battle’ prevails, weapons such as antlers are gradually 
evolved, because the individual possessing these in the most 
highly developed form will be most successful in the struggle 
1907 June tr. 
