56 
ISLAND LIFE. 
[part I. 
will probably die out altogether, and thus leave room for others 
to increase, or for new forms to occupy their places. 
But the change will most likely affect even flourishing species 
in different ways, some beneficially, others injuriously. Or, 
again, it may affect a great many injuriously, to such an 
extent as to require some change in their structure or habits 
to enable them to get on as well as before. Now “variation” 
and the “ struggle for existence ” come into play. All the 
weaker and less perfectly organised individuals die out, while 
those which vary in such a way as to bring them into more 
harmony with the new conditions constantly survive. If the 
change of conditions has been considerable, then, after a few 
centuries, or perhaps even a few generations, one or more new 
species will be almost sure to be formed. 
Variation in Animals . — To make this more intelligible to 
those who have not considered the subject, and to obviate the 
difficulty many feel about “ favourable variations occurring at 
the right time,” it will be well to discuss this matter a little 
more fully. Few persons consider how largely and universally 
all animals are varying. We know, however, that in every 
generation, if we could examine all the individuals of any 
common species, we should find considerable differences, not 
only in size and colour, but in the form and proportions of all 
the parts and organs of the body. In our domesticated animals 
we know this to be the case, and it is by means of the continual 
selection of such slight varieties to breed from that all our ex- 
tremely different domestic breeds have been produced. Think 
of the difference in every limb, and every bone and muscle, and 
probably in every part, internal and external of the whole body, 
between a greyhound and a bull-dog ! Yet, if we had the whole 
series of ancestors of these two breeds before us, we should pro- 
bably find that in no one generation was there a greater differ- 
ence than now occurs in the same breed, or sometimes even the 
same litter. It is often thought, however, that wild species do 
not vary sufficiently to bring about any such change as this in 
the same time ; and though naturalists are well aware that this 
is a mistake, it is only recently that they are able to adduce 
positive proof of their opinion. 
