218 THE ORIGIN OF SPECIES 



than in young snakes acquiring in their upper jaws, as Owen has 

 remarked, a transitory sharp tooth for cutting through the tough 

 egg-shell. For if each part is liable to individual variations at all 

 ages, and the variations tend to be inherited at a corresponding 

 or earlier age — propositions which cannot be disputed — then the 

 instincts and structure of the young could be slowly modified as 

 surely as those of the adult; and both cases must stand or fall to- 

 gether with the whole theory of natural selection. 



Some species of Molothrus, a widely distinct genus of American 

 birds, allied to our starlings, have parasitic habits like those of the 

 cuckoo; and the species present an interesting gradation in the 

 perfection of their instincts. The sexes of Molothrus badius are 

 stated by an excellent observer, Mr. Hudson, sometimes to live 

 promiscuously together in flocks, and sometimes to pair. They 

 either build a nest of their own or seize on one belonging to some 

 other bird, occasionally throwing out the nestlings of the stranger. 

 They either lay their eggs in the nest thus appropriated, or oddly 

 enough build one for themselves on the top of it. They usually sit 

 on their own eggs and rear their own young; but Mr. Hudson says 

 it is probable that they are occasionally parasitic, for he has seen 

 the young of this species following old birds of a distinct kind and 

 clamoring to be fed by them. The parasitic habits of another spe- 

 cies of Molothrus, the M. bonariensis, are much more highly de- 

 veloped than those of the last, but are still far from perfect. This 

 bird, as far as it is known, invariably lays its eggs in the nests of 

 strangers; but it is remarkable that several together sometimes 

 commence to build an irregular untidy nest of their own, placed 

 in singular ill-adapted situations, as on the leaves of a large thistle. 

 They never, however, as far as Mr. Hudson has ascertained, com- 

 plete a nest for themselves. They often lay so many eggs — from 

 fifteen to twenty — in the same foster-nest, that few or none can 

 possibly be hatched. They have, moreover, the extraordinary 

 habit of pecking holes in the eggs, whether of their own species or 

 of their foster-parents, which they find in the appropriated nests. 

 They drop also many eggs on the bare ground, which are thus 

 wasted. A third species, the M. pecoris of North America, has ac- 

 quired instincts as perfect as those of the cuckoo, for it never lays 

 more than one egg in a foster-nest, so that the young bird is se- 

 curely reared. Mr. Hudson is a strong disbeliever in evolution, but 

 he appears to have been so much struck by the imperfect instincts 

 of the Molothrus bonariensis that he quotes my words, and asks, 

 "Must we consider these habits, not as especially endowed or ere- 



