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SCIENCE 



[N. S. Vol. XL. No. 1026 



a degree this is true of biology, and as a 

 chief characteristic of modem evolutionary 

 thought we must confess also to a deep but 

 irksome humility in presence of great vital 

 problems. Every theory of evolution must 

 be such as to accord with the facts of physics 

 and chemistry, a primary necessity to which 

 our predecessors paid small heed. For 

 them the unknown was a rich mine of pos- 

 sibilities on which they could freely draw. 

 For us it is rather an impenetrable moun- 

 tain out of which the truth can be chipped 

 in rare and isolated fragments. Of the 

 physics and chemistry of life we know next 

 to nothing. Somehow the characters of 

 living things are bound up in properties of 

 colloids, and are largely determined by the 

 chemical powers of enzymes, but the study 

 of these classes of matter has only just 

 begun. Living things are found by a sim- 

 ple experiment to have powers undreamed 

 of, and who knows what may be behind ? 



Naturally we turn aside from general- 

 ities. It is no time to discuss the origin of 

 the Mollusca or of Dicotyledons, while we 

 are not even sure how it came to pass that 

 Primula ohconica has in twenty-five years 

 produced its abundant new forms almost 

 under our eyes. Knowledge of heredity 

 has so reacted on our conceptions of varia- 

 tion that very competent men are even 

 denying that variation in the old sense is a 

 genuine occurrence at all. Variation is 

 postulated as the basis of all evolutionary 

 change. Do we then as a matter of fact 

 fimd in the world about us variations occur- 

 ring of such a kind as to warrant faith in 

 a contemporary progressive evolution? 

 Till lately most of us would have said 

 "yes" without misgiving. We should have 

 pointed, as Darwin did, to the immense 

 range of diversity seen in many wild spe- 

 cies, so commonly that the difficulty is to 

 define the types themselves. Still more con- 

 clusive seemed the profusion of forms in 



the various domesticated animals and 

 plants, most of them incapable of existing 

 even for a generation in the wild state, and 

 therefore fixed unquestionably by human 

 selection. These, at least, for certain, are 

 new forms, often distinct enough to pass 

 for species, which have arisen by variation. 

 But when analysis is applied to this mass 

 of variation the matter wears a different 

 aspect. Closely examined, what is the 

 "variability" of wild species? "What is 

 the natural fact which is denoted by the 

 statement that a given species exhibits much 

 variation? Generally one of two things: 

 either that the individuals collected in one 

 locality differ among themselves ; or perhaps 

 more often that samples from separate 

 localities differ from each other. As direct 

 evidence of variation it is clearly to the 

 first of these phenomena that we must have 

 recourse — the heterogeneity of a popula- 

 tion breeding together in one area. This 

 heterogeneity may be in any degree, rang- 

 ing from slight differences that systematists 

 would disregard, to a complex variability 

 such as we find in some moths, where there 

 is an abundance of varieties so distinct that 

 many would be classified as specific forms 

 but for the fact that all are freely breeding 

 together. Naturalists formerly supposed 

 that any of these varieties might be bred 

 from any of the others. Just as the reader 

 of novels is prepared to find that any kind 

 of parents might have any kind of children 

 in the course of the story, so was the evolu- 

 tionist ready to believe that any pair of 

 moths might produce any of the varieties 

 included in the species. Genetic analysis 

 has disposed of all these mistakes. We have 

 no longer the smallest doubt that in all 

 these examples the varieties stand in a regu- 

 lar descending order, and that they are 

 simply terms in a series of combinations of 

 factors separately transmitted, of which 

 each may be present or absent. 



