THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 411 



are known, and such were taken by Mivart and other zoologists to 

 prove that species do not originate by gradual change, but abruptly 

 or by " extraordinary births," a view quite similar to the more recent 

 theories of nuitations." but distinctly more practical, because the 

 " iiiutations "' of plants which are the basis of the inferences of Pro- 

 fessor De Vries are not ])repotent but " recessive,''' presumably because 

 they do not represent true genetic variations, Init are symptoms of 

 what may be described as an evolutionary debility, due to inbreeding. 

 The disappearance of mutative characters when the new variations 

 are crossed with the parent form or with each other is merely the 

 recovery, as it were, of the health of the species when the abnormal 

 condition of inl)reeding has been removed, as shown so conclusively 

 m Darwin's well-known experiments with jiigeons, and confirmed by 

 an abundance of similar facts. Though differently interpreted, 

 many other facts supporting this view" were collected by Darwin, who 

 summarized the results of his studies of Ipomea, Digitalis, Origanum, 

 Viola, Bartonia, Canna, and the common cabbage and pea, as follows: 



The most iniportant conclusion at which I have arrived is that the mere act 

 of crossing by itself does no good. The good depends on the individuals which 

 are crossed diffei-ing slightly in constitution, owing to their progenitors having 

 been subjected during several generations to slightly different conditions, or to 

 what we call in our ignorance " spontaneous variations." 6 



Differences between the plants of different habitats mean also dif- 

 ferent lines of descent and attendant variations, and the beneficial re- 

 sults of bringing these together may be explained by reference to 

 symbasis rather than to the " slightly different conditions." 



While it may not be insisted that species, as described and named 

 by systematists, are never originated by '' extraordinary births " or 

 from •' mutations," both suppositions are obviously improbable as gen- 

 eral explanations. Mutations are seldom fitted to survive because 

 they are less vigorous and less fertile than the parent type, so that 

 they must be segregated at once in order to be preserved. And even 

 prepotent variations have no necessary connection with the origina- 

 tion of species, since however rapidly the characters of a species 

 might change, it would still l)e the same species until a subdivision 

 had taken place. The more a species evolves the more different from 

 its relatives it becomes, and the more satisfactory for the })ur2)oses of 



a Mr. William K. Maxon, of the U. S. National Museum, calls my attention to a 

 very early announcement of the mutation theory. 



"The truth is that species, and perhaps gen(M-a :ilso, arc forming in organized 

 beings by gradxial deviations of shapes, forms, :nid organs, taking i)lace in the 

 lapse of time. There is a tendency to deviations and nuitations through plants 

 and animals by gradual stei)S at remote irregular periods." Rafinesque, 18.''.2. 

 Quoted in Journal of Botany, .30 : 810. 



''Darwin, The Effects of Cross and Self Fertilization in the N'egetable King- 

 dom, p. 27. New York, 1895. 



