INTRODUCTORY 15 



of hybrids by parthenogenetic reproduction, a polymorphism 

 due to the continued presence of individuals representing various 

 combinations of Mendelian allelomorphs, partly also the tran- 

 sient effect of alteration in external circumstances, we see how 

 cautious we must be in drawing inferences as to the indefiniteness 

 of specific limits from a bare knowledge that intermediates exist. 

 Conversely, from the accident of collocation or from a mislead- 

 ing resemblance in features we deem essential, forms genetically 

 distinct are often confounded together, and thus the divergence 

 of such forms in their other features, which we declare to be 

 non-essential, passes as an example of variation. Lastly, and 

 this is perhaps the most fertile of all the sources of confusion, 

 the impression of the indefiniteness of species is created by the 

 existence of numerous local forms, isolated geographically from 

 each other, forms whose differences may be referable to any one 

 of the categories I have enumerated. 



The advance has been from many sides. Something has 

 come from the work of systematists, something from cultural 

 experiments, something from the direct study of variation as it 

 appears in nature, but progress is especially due to experimental 

 investigation of heredity. From all these lines of inquiry we 

 get the same answer; that what the naturalists of fifty years 

 ago regarded as variation is not one phenomenon but many, 

 and that what they would have adduced as evidence against 

 the definiteness of species may not in fact be capable of this 

 construction at all. 



If we may once more introduce a physical analogy, the dis- 

 tinctions with which the systematic naturalist is concerned in 

 the study of living things are as multifarious as those by which 

 chemists were confronted in the early days of their science. 

 Diversities due to mechanical mixtures, to allotropy, to differences 

 of temperature and pressure, or to degree of hydration, had all 

 to be severally distinguished before the essential diversity due 

 to variety of chemical constitution stood out clearly, and I 

 surmise that not till a stricter analysis of the diversities of animals 

 and plants has been made on a comprehensive scale, shall we 

 be in a position to declare with any confidence whether there is 



