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2 Darwin explained and exemplified very fully and convincingly the working of his principle of "the natural selection of numerous successive, slight, favourable variations"; and he considered this to be the chief factor. But he allowed, at the same time, that the Lamarckian factors had an important, though a somewhat subordinate part to play in the origin of species. After his time a school of biologists, consisting of Weismann and his followers, has arisen which rejects the Lamarckian factors and maintains that natural selection acting upon fortuitous variations, is sufficient in itself to explain organic evolution. Another school, to which Herbert Spencer and Haeckel belong and which finds a large support in America especially amongst the paleontologists, while admitting that natural selection has played an important part in evolution, emphazises the relative importance of the environmental and functional changes produced in organisms and lays stress on the transmission of acquired character. Some even go so far as to throw the whole burden of evolution on the Lamarckian factors.

A third school, represented by Nageli, Henslow, Eimie and others, rejects the view that variations are haphazard, believing on the contrary that they occur in a determinate direction due to some force which is thought to be at work within the organism and prior to the operation of natural selection. The nature of this force is not very clear and the different writers probably do not always mean the same thing: it has been variously called