In his preface Lamarck had already declared that "the thread which gives us a clue to the causes of the various phenomena of animal organization, in the manifold diversity of its developments, is to be found in the fact that Nature conserves in offspring all that their life and environments has developed in parents." Heredity--"the hidden bond of common descent"--tempered with the modifications induced by changed habits--which changed habits are due to new conditions and surroundings--this with Lamarck, as with Buffon and Dr. Darwin, is the explanation of the diversity of forms which we observe in nature. He now goes on to support this--briefly, in accordance with his design--but with sufficient detail to prevent all possibility of mistake about his meaning. "In the same climate differences in situation, and a greater or less degree of exposure, affect simply, in the first instance, the individuals exposed to them; but in the course of time, these repeated differences of surroundings in individuals which reproduce themselves continually under similar circumstances, induce differences which become part of their very nature; so that after many successive generations, these individuals, which were originally, we will say, of any given species, become transformed into a different one."[231] "Let us suppose that a grass growing in a low-lying meadow gets carried by some accident to the brow of a neighbouring hill, where the soil is still damp enough for the plant to be able to exist. Let it live here for many generations, till it has become thoroughly accustomed to its position, and let it then gradually find its way to the dry and almost arid soil of a mountain side; if the plant is able to stand the change and to perpetuate itself for many generations, it will have become so changed that botanists will class it as a new species."[232] "The same sort of process goes on in the animal kingdom, but animals are modified more slowly than plants."[233] The sterility of hybrids, to which Mr. Darwin devotes a great part of the ninth chapter of his 'Origin of Species,'[234] is then touched on--briefly, but sufficiently--as follows:--