THE EVOLUTIONARY RTCNTFTrA-NOE OF SPECIES. 411 



are known, and snch Avoro taken hy Mivart and other zoolo<jfists to 

 prove that species do not originate by gradual change, hut ahi-uptly 

 or by " extraoi'dinarv births,"" a view quite simihir to the nioi-e recent 

 theories of nnitalions," but distinctly more practical, because the 

 " mutations " of plants which arc the basis of the inferences of Pro- 

 fessor De Vries are not pre})otent but " recessive," presumably because 

 they do not represent true genetic variations, but 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 Avith each other is merely the 

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

 condition of inbreeding has been removed, as shown so conclusively 

 in Darwin's well-known experiments with pigeons, 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 luost iiuitortant conclusion at which I have arrived is that the mere act 

 of crossing hy itself docs no good. The good depends on the individuals which 

 ;ire crossed differing 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." & 



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 l)e 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 be 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 purposes of 



« Mr. William H. Maxon, of the U. S. National Museum, calls niy attention to a 

 very early announcement of the mutation theory. 



"The truth is that species, and i)erhai)s gener:\ also, are forming in organized 

 beings by gi-adual deviations of shapes, forms, and organs, taking place in the 

 lapse <tf time. There is a tendency to deviations and nnitations through plants 

 and animals by gradual steps at remote irregular jteriods." lia(1nes(|ue. IS.'i2. 

 Quoted in Journal of Botany, .30::?10. 



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

 dom, p. 27. New York, 1895. 



