xv THE PHILOSOPHY OF ZOOLOGY 633 



frequently and extensively, especially in the form and markings of 

 the shell ; and of some of the species which have been most com- 

 pletely studied in this respect a very large number of more or less 

 strongly marked varieties have been recorded. Many of the 

 Crustacea are also extremely variable in coloration and in the 

 length and proportions of the various appendages. But, among 

 the Arthropoda, it is in the Insecta, and more especially the 

 Lepidoptera, that we find the most striking instances of variation. 

 In the Vertebrata, also, variations in colour and proportions, as 

 well as in internal organs, occur frequently in all classes. 



Mutations. It has been shown by de Vries that variations are 

 not always of a comparatively minute character. According to 

 de Vries, in addition to the small fluctuations on which Darwin 

 mainly relied for the phylogenetic development of organisms, there 

 are others which occur comparatively rarely, and are of a much 

 more striking character. These larger variations, which de Vries 

 distinguishes by the name of mutations, take the form of the 

 sudden appearance of differences equivalent to the formation per 

 saltum of new species ; and it is by the successive appearance of 

 such steps or leaps, and not by the more gradual process of Dar- 

 winian variation, that, according to de Vries, progress from the 

 lower towards the higher is effected. The new forms developed 

 in this sudden way may live side by side with the old, and thus 

 isolation is not necessary for their perpetuation. 



Natural Selection. According to Darwin's theory of Natural 

 Selection, nature, i.e., the conditions under which the organism 

 exists, selects certain variations as they arise, very much as the 

 breeder or the gardener selects variations in domestic animals or 

 cultivated plants. Let us see how' this selection is carried on. 

 We have seen that there is going on, on all sides, a struggle for 

 existence. It is at first difficult to realise the intensity of this 

 struggle, for there is little appearance of it on the surface. If we 

 consider, however, that a large proportion of living things prey on 

 living things of other groups, and when we bear in mind the 

 extremely small proportion which, in most cases, the surviving 

 individuals of any group bear to the number of young produced, 

 we come to understand that this struggle for existence must be 

 general and intense. 



Now in the case of a species living under tolerably uniform and 

 stable conditions as regards climate, food-supply, and the like, the 

 effects of this struggle will be the survival of the fittest. Of the 

 young produced, only a small proportion (in most cases) reach 

 maturity ; some of these surviving forms have survived, perhaps, 

 because they have happened to escape being preyed upon by 

 enemies, while others have succumbed ; but there can be little 

 doubt that, in the long run, such individuals will survive as are 

 best fitted to cope with the conditions to which they are subjected 



