I. Haeckel and His Opponents

As to this “how,” it was the numerous breeding experiments which he tried, after his return home, with pigeons, fowls, dogs, rabbits, and garden plants that enlightened Darwin. He saw from them in how high a degree there lies in organic forms the possibility of continually modifying themselves in the course of their reproduction. It is possible, by creating artificial conditions, to obtain from a given form after a few generations new kinds, which differ much more from each other than do those in Nature, whose difference is regarded as so great that one inclines to ascribe to each a special underlying plan of organisation. As is well known, the breeder utilises this variability of kinds to bring about the development of such forms of domesticated organisms as correspond with his intentions. He endeavours to create the conditions which guide the variation in a direction answering his purpose. If he seeks to breed a kind of sheep with specially fine wool, he seeks out among his flock those individuals which have the finest wool. These he allows to breed. From among their descendants he again selects for further breeding those which have the finest wool. If this is carried on through a series of generations, a species of sheep is obtained which differs materially from its ancestors in the formation of its wool. The same thing can be done with other characteristics of living creatures. From these facts two things become obvious: that organic forms have a tendency to vary, and that they pass on the acquired modifications to their descendants. Owing to this first property of living creatures, the breeder is able to develop in his species certain characteristics that answer his purposes; owing to the second, these new characteristics are handed on from one generation to the next.