130 HEREDITY AND SELECTION IN SOCIOLOGY 



tiated forms whicli we usually associate with tlie idea of species, 

 but a series of gradual variations merging one into another. But 

 if we take the parent species and compare it with the most distant 

 species, we find at once that there is an enormous difference, 

 which justifies our regarding them as distinct species. 



The interpretation of this long chain of gradually increasing 

 differentiations is to be found in the fact that a primitive Tertiary 

 species has spread gradually through the whole length and 

 breadth of the island, differentiating itself from the original type 

 in proportion to the distance between a new habitat and the 

 original head-quarters. We have here before us the different 

 stages of a progressive phyletic evolution, not in time alone, but 

 also in space ; the successive stages of that evolution coexist in 

 space before us. It is as if we had Darwin's diagram before our 

 eyes, and could trace the successive steps of evolution.^ 



We have studied several problems of evolution and heredity, 

 and our general conclusion has been that selection is not only the 

 main, but the exclusive, factor in bringing about evolution. 

 Selection, as we have seen, begins already among the primordial 

 living elements, among the biophors and determinants of the 

 germ-plasm ; so that even those characters which possess only a 

 morphological, as distinct from a biological, value are likewise 

 the result of a selection exercised among these primitive elements 

 of the germ. We have examined a few cases which have been 

 especially urged in support of the Lamarckian hypothesis, and 

 we found that they do not really prove any transmission of the 

 results of use or disuse, nor the postulated reaction of the soma 

 on the germinal material. In regard to hereditary instincts, 

 we saw that selection preserved those modes of activity which 

 are indispensable to the life of the species. We saw, again, that 

 amphimixis, as also asexual reproduction, is due ahke in its 

 origin and development to selection ; for every mode of repro- 

 duction is an adaptation, and is consequently the work of selec- 

 ^ The Origin of Species, p. 140. 



