ISOLATION 181 



only when they have had some means of keeping themselves 

 apart. And if a group is to keep apart, its units must have the 

 power of holding together, since, however necessary the isolation 

 of the species, an isolation of units would be fatal. 



Difficult as it is, from the nature of the case, to obtain direct 

 evidence of the working of the system of recognition marks, 

 there is nevertheless some. Cattle certainly notice colour differ- 

 ences, and when allowed to run wild they will sometimes separate 

 into herds, which, except in respect of colour, have apparently no 

 distinguishing characteristic. Again, it is a remarkable fact that 

 among wild animals, except for very slight deviations, bilateral 

 symmetry in coloration is always adhered to. Whereas in many 

 of our domestic cows the two flanks often present quite a different 

 appearance. Such vagaries are possible in the tame breeds where 

 man, instead of nature, selects. Among wild animals they must 

 occasionally arise and be rejected. Very possibly an instinct, 

 that serves to preserve recognition marks as the cement of the 

 breed, prompts wild cattle to drive such monstrosities from the 

 herd. 



Birds, I think, enable us better than any other class to under- isolation 

 stand this subject. When they, or rather their progenitors, were j^ds^ 

 still in a reptilian stage, with very limited powers of locomotion, 

 there would not be great risk of their failing to find each other. 

 Even where their swimming powers are great, it is difficult for 

 reptiles to wander far. They cannot frequent the open sea, since 

 they must be within reach of the land. In the Nile, as they swim 

 up and down, crocodiles must meet each other, or they can find 

 their fellows as they lie basking on the banks. Amphibians also 

 have limited powers of locomotion. For them, as for reptiles, 

 the wide seas are an impossible habitat, since, being air-breathers 

 in their mature state, they must, unless they have some very 

 special adaptations to aquatic life, be able to land when they need 

 rest. This remains a fact, though frogs can live at the bottom 

 of a pond in a torpid state during the winter, breathing sluggishly 

 through the skin. To reptiles and amphibians, then, call-notes 

 cannot be so important as to that class whose powers enable them 

 to range over the whole globe. Nevertheless, frogs have a very 



