viii 
INTEODIJCTION. 
but it is, we believe, an admitted fact that all good breeds require constant supervision and 
occasional introduction of fresh blood to prevent deterioration. 
Though it is allowable to assume that variation is possible, and that many even of natural 
species, especially local varieties, may have had a common origin, the position is greatly 
weakened by attempting to trace for it a universal application; and though many of the 
wonderful instincts to be found in wild animals and in the inferior forms of life are very pro- 
bably referable to the hereditary transmission of acquired habits, on this account to take for 
granted that all are acquired and none inherent raises a host of difficulties at once ; and it 
is here that some of the arguments in support of the hypothesis of modified descent appear 
overstrained : for instance, in his seventh chapter, on instincts, Mr. Darwui attributes the 
origin of the well-known parasitic habits of our European Cuckoo to the gradual prevalence 
of an occasional and accidental action of an ancestor, the progeny which imitated her, through 
a supposed advantage thereby obtained in the struggle for existence, gradually exterminating 
the progeny which did not, till the instinct became universal. Now this can hardly account 
satisfactorily for the entire prevalence of the habit in one species only ; for the advantage 
gained is at best so trifling that it would constantly be overbalanced by indi^ddual variations in 
strength and intelligence quite independent of the habit: and the only other inference to be 
drawn from it is too flattering to the Cuckoos as a body to be accepted ; for it leads us to infer 
that in the whole race not a single individual is to be found sufficiently short-sighted and 
imprudent to neglect so trifling an advantage, while among the Swallows and other 
migratory birds not a single individual, from the beginning of time until now, has had the 
wit to hit on so simple an expedient ; for if one had hit on it, there is as much reason to 
assume that the cunning Swallow's progeny would have exterminated the others as there 
is in the case of the Cuckoos. 
It will be found difficult, also, to reconcile this universal prudence on the part of the 
Cuckoos with the theory of the common descent of all organized beings, when we consider 
the infinite variety of form and intelligence displayed in the animated creation. We know 
that many a wise man begets a fool, and doeth it to his sorrow ; and why the Cuckoos should 
enjoy such complete immunity we cannot say. It is true that the rejection of this explana- 
tion lands us in the dilemma of believing that an Allwise Creator endowed a portion of His 
creatures with an instinct apparently subversive of the maternal affections ; but as even on the 
other hand we must admit that this instinct has grown out of the unrestrained action of His 
law, it is not a wide step further to believe that it was from the beginning a peculiarity of 
this species, and that we have as yet failed to fathom the true intention. 
Again, to take another instance, the individuals of the species Tinnunculus alaudarius, 
TJrrua coromanda, and many other Raptorial birds, which occasionally make use of the 
deserted nests of Crows and other large birds, as is well known to observers of Nature, 
should have gained an equally great, if not greater, advantage in the struggle for existence, 
in the increased leisure obtained to seek for food, and the habit ought long ere now to have 
become universal with them. In the same way the putting on of dull green or brown 
plumage, so essential to small forest-birds to protect them from the observation of their 
