Il6 THE FOUNDATIONS OF ZOOLOGY 



Magnus that they rain down from the clouds, where they are devel- 

 oped from decomposing exhalations impregnated with the semen 

 of rats. 



It is easy to understand how birds near the northern limit of 

 their range may invade the territory of those whose home is a little 

 further south, and compete with them for food as this becomes 

 scarce on the approach of winter, and how this movement spreads 

 until all the members of the species are involved, although many 

 of these might have been able to satisfy all the necessities of life 

 for some time longer in their breeding grounds, if they had been 

 undisturbed. 



We have noted that this has commended itself to northern natu- 

 ralists as a sufficient reason for the acquisition of the migratory 

 habit, and that the fondness for their birthplace which is so strongly 

 developed in birds has been thought enough to draw them back ; 

 but the love of home is itself a result of natural selection, and the 

 necessity for finding safe places for the eggs and young enough 

 to account for the origin of migration without the aid of geological 

 changes. 



Even if we know little as to the means by which birds find their 

 way over land and water, we know as a fact that they are able to 

 do so ; and we also know that the instinct which leads them to 

 seek safe places for their nests is so strongly implanted in their 

 nature that centuries of domestication weaken it but little, for it is 

 still as strong in the guinea fowl and the turkey and the hen as it 

 is in wild birds. As birds of powerful flight have a range of choice 

 in the selection of places for their nests which is almost as wide as 

 the earth itself, it is not surprising that the continual destruction 

 of those born in the least safe nests has at last resulted in the sur- 

 vival of the ones which build their nests thousands of miles away 

 from their ancestral home. 



While most writers on the subject have thought that migration 

 had its origin in an annual journey which, while short, was definite 

 for all the members of the species, and while they have felt forced 

 to call in the aid of geology to account for the gradual separation 

 of the two termini, and the length of the journey from one to the 

 other, the hypothesis of geological change seems gratuitous and 

 unnecessary, since the known habits and instincts and needs of 



