HIGH SCHOOL ZOOLOGY. 



283 



ponding differences of structure ; such are the association of 

 underground life with the absence of eyes and of colour (II, 79 ; 

 III, 12) ; and that of the shape of the body with the medium 

 and mode of locomotion (Fig. 194). These offer a very tempt- 



Fiff. 194.— Han^ Seal. Phoca groenlandica (from Brehm). 



ing field for investigation, and cause us to enquire how changes 

 in surrounding conditions and in habits affect animal life. 



19. Heference has ah'eadybeen made to the fact that organisms 

 tend to increase in number in geometrical progression. Some- 

 times countless eggs are produced by a single individual, but in 

 such cases few of these reach maturity, and it is onl}- now and 

 then that we are startled by the disproportionate development 

 of some one species, disturbing the course of Nature. When, 

 however, one generation exceeds the previous one, even in the 

 proportion of two to one, it is evident that there will soon be a 

 competition or struggle between the individuals for suitable 

 food, and that the strongest and best adapted to survive will be 

 those, on the whole, that do survive. Along with this we have 

 to take into account the tendency of organisms to vary and to 

 transmit their peculiarities to their descendants (§ 7). Those 

 individuals whose organs vary in such a way as to adapt them 

 better, however little, to their special circumstances of life, are, 



