66 



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 neio 

 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 difi^er- 

 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. 



