368 MENTAL EVOLUTION IN ANIMALS. 



observed* that ants ceased their laborious task of daily 

 moving their eggs to and from the surface according to the 

 heat of the sun, when they had built their nest between the 

 two cases of a bee-hive, where a proper and equable tempe- 

 rature was provided. 



Now let us suppose that the conditions of life favoured 

 the extension of a bird of this Family, whose eggs were 

 hatched by the solar rays alone, into a cooler, damper, and 

 more wooded country : then those individuals which chanced to 

 have the accumulative propensity so far modified as to prefer 

 more leaves and less sand, would be favoured in their exten- 

 sion ; for they would accumulate more vegetable matter, and 

 its fermentation would compensate for the loss of solar heat, 

 and thus more young birds would be hatched which might as 

 readily inherit the peculiar accumulative propensity of their 

 parents as our breeds of dogs inherit a tendency to retrieve, 

 another to point, and another to dash round its prey. And 

 this process of natural selection might be continued, till the 

 eggs came to be hatched exclusively by the heat of fermen- 

 tation ; the bird, of course, being as ignorant of the cause of 

 the heat as of that of its own body. 



In the case of corporeal structures, when two closely 

 allied species, one for instance semi-aquatic and the other 

 terrestrial, are modified for their different manners of life, 

 their main and general agreement of structure is due, according 

 to our theory, to descent from common parents ; and their 

 slight differences to subsequent modification through natural 

 selection. So when we hear that the thrush of South America 

 (T. Falklandicus), like our European species, lines her nest in 

 the same peculiar way with mud, though, from being sur- 

 rounded by wholly different plants and animals, she must be 

 placed under somewhat different conditions ; or when we 

 hear that in North America the males of the kitty wrens.f 

 like the male of our species, have the strange and anomalous 

 habit of making " cock-nests," not lined with feathers, in 

 which to shelter themselves ; — when we hear of such cases, 

 and they are sufficiently numerous in all classes of animals, 

 we must attribute the similarity of the instinct to inheritance 

 from common progenitors, and the dissimilarity, either to 



* Kirby and Spencc, Inlroi. to Entomol., vol. ii, p. 519. 

 t Peabody in Boston Journ. Nat. Hist., vol. iii, p. lit. For our British, 

 species seo Macgillivray, Brit. Birds, vol. iii, p. 23. 



