608 THE POPULAR SCIENCE MONTHLY. 



pearing in May and leaving again in August. The former bird is large 

 and strong enough to capture other grown birds and mice, and can 

 thus always find a bounteous table spread for itself, while the others, 

 confined by their more limited powers to smaller creatures, have to 

 go away when, with the end of summer, these fail. The carrion- 

 crow, to which everything that it is only possible to swallow furnishes 

 a delicious feast, is not troubled by the severity of winter ; but the 

 rook, whose bill of fare is more limited, is in no condition to endure 

 the scarcity of the winter months, and, therefore, on the approach of 

 fall, it flies in large flocks over the Alps and Pyrenees, to the luxuriant 

 fields of Southern Europe and Africa. 



'While this brief glance at the connection between the yearly migra- 

 tions of birds and their food-supply must suffice for that point, the 

 consideration of the distribution and accidental wanderings of these 

 remarkable creatures, so far as it is related to their commissariat, de- 

 mands more attention. It is evident that an animal species can live 

 and thrive only when its accustomed provision, or some substitute 

 similar in character to it, is present ; where this is wanting, it can not 

 gain a permanent footing ; or it must adapt itself to its new relations, 

 undergo gradual modifications in its habits and organization, and thus 

 in the course of time become another and new variety. But wherever 

 it can find its accustomed subsistence in the usual quantity and season, 

 it will readily make itself at home without having to undergo any 

 modification, unless it is provoked by other causes. If we ask what 

 creatures — what birds, in the present case — have under particular cir- 

 cumstances the greatest chance for a wide distribution, the answer will 

 be that they are those which, like the crow, the thrush, and the true 

 shrike, are least particular in the choice of what they eat ; and next, 

 those whose appropriate food is most widely diffused. But the more 

 closely a bird is adapted to particular kinds of food, and the more lim-' 

 ited the circle of its distribution, the narrower will be the field of its 

 residence. This is the case, not only with species, but in a wider sense 

 with genera and families, and it is not to be overlooked that very 

 widely distributed species are also frequently permanent residents. 

 The most uniformly distributed animals under all conditions of cli- 

 mate and season are vertebrates, especially fishes and the smaller land- 

 mammalia, and also some insects ; and they also exhibit less important 

 and diversified variations, and demand less on the part of their pursu- 

 ers, than insects. Next to these are certain invertebrates of fresh and 

 salt water ; and among these the shore-species, particularly within 

 their seasonal limits, display a great uniformity in all parts of the 

 earth. Birds of prey, except the carrion-eaters, live on land-verte- 

 brates. Three of their four families include quite or nearly cosmopol- 

 itan gen >ra, and, what is rare among land-birds, only three species are 

 wanting in very small districts. Fish and water animals constitute, 

 with few exceptions, the food of the large groups known as the waders 



