INTRODUCTION 3 



relations, their geographical distribution, geological 

 succession, and other such facts, might come to the 

 conclusion that each species had not been independently 

 created, but had descended, like varieties, from other 

 species. Nevertheless, such a conclusion, even if well 

 founded, would be unsatisfactory, until it could be shown 

 how the innumerable species inhabiting this world have 

 been modified, so as to acquire that perfection of struc- 

 ture and coadaptation which most justly excites our 

 admiration. Naturalists continually refer to external 

 conditions, such as climate, food, etc., as the only pos- 

 sible cause of variation. In one very limited sense, as 

 we shall hereafter see, this maybe true; but it is pre- 

 posterous to attribute to mere external conditions the 

 structure, for instance, of the woodpecker, with its feet, 

 tail, beak, and tongue, so admirably adapted to catch 

 insects under the bark of trees. In the case of the 

 mistletoe, which draws its nourishment from certain 

 trees, which has seeds that must be transported by 

 certain birds, and which has flowers with separate sexes 

 absolutely requiring the agency of certain insects to 

 bring pollen from one flower to the other, it is equally 

 preposterous to account for the structure of this parasite, 

 with its relations to several distinct organic beings, by 

 the effects of external conditions, or of habit, or of the 

 volition of the plant itself. 



The author of the Vestiges of Creation would, I 

 presume, say that, after a certain unknown number of 

 generations, some bird had given birth to a woodpecker, 

 and some plant to the mistletoe, and that these had 

 been produced perfect as we now see them ; but this 

 assumption seems to me to be no explanation, for it 

 leaves the case of the coadaptations of organic beings 

 to each other and to their physical conditions of life, 

 untouched and unexplained. 



It is, therefore, of the highest importance to gain a 

 clear insight into the means of modification and co- 

 adaptation. At the commencement of my observations 

 it seemed to me probable that a careful study of domes- 

 ticated animals and of cultivated plants would offer the 



