332 A HISTORY OF BIRDS . 



of species is profound. Here the victors necessarily have some 

 physical, some structural, advantage over the vanquished. 

 They prevail because better equipped and better adapted for 

 their environment. It was supposed that this superiority owed 

 its existence to the selection of structural variations in the 

 right direction. Since no two individuals of a brood are exactly 

 alike, it seemed obvious that the variations hereby displayed 

 would confer on some individuals a greater fitness for their en- 

 vironment, while in others it would have the reverse effect, and 

 these would sooner or later disappear. The unfit, by one 

 agency or another, in short, became weeded out. 



But it is now recognised that a distinction must be drawn 

 between individual somatic variations, variations due to feeding 

 for example, and congenital, inborn, innate variations. That 

 is to say, between what we may call purely individual, physical 

 variations due more or less entirely to post-embryonic con- 

 ditions, and variations which have their origin in peculiarities 

 of the germ plasm, which are alone transmitted to offspring. 

 Variations of this latter kind, we are now beginning to realise, 

 have a cumulative effect ; they show a decided tendency to 

 increase in each successive generation, and are not, as was sup- 

 posed, swamped by inter-crossing. The sum of all the char- 

 acters essential to the survival of an individual in the struggle 

 for existence, a struggle against the inanimate no less than the 

 animate environment, must attain a certain minimum standard 

 to ensure survival under the most favourable circumstances. 

 But where the struggle becomes intensified, as by the increase 

 of the species or competition in any form, that standard is 

 raised, and with this comes a change in the facies of the species, 

 or evolution. 



This being so then, what is known as intra-selection must 

 be allowed more attention than has hitherto been accorded it. 



The congenital variations, to which reference has just been 

 made, are variations which occur in the several organs, and 

 parts of organs, which make up the individual. And these 

 variations, as we have just remarked, once started, tend to go 

 on increasing in each successive generation, so that the survival 

 of this or that peculiarity, be it external or internal, follows on 

 the same laws as the survival of this or that species. We may, 

 in short, in following the history of the development of such and 



