June 7, 1907] 



SCIENCE 



883 



is a perennial successon of like individ- 

 uals." An equally high botanical au- 

 thority has said that a species is a judg- 

 ment. And this also is true. Species and 

 other categories of classification are more 

 or less arbitrary distinctions, made for con- 

 venience in classification of our knowledge. 

 Hence in a given case, the question whether 

 two different forms are to be regarded as 

 two different species or not, is in part a 

 matter of individual judgment. If Dar- 

 win's view is correct, that new species may 

 originate by the gradual accumulation of 

 exceedingly minute differences, there could 

 be no line of demarcation between species 

 provided we could have all the transitional 

 forms. Only where the transitional forms 

 had disappeared, or the new forms had mi- 

 grated to a new region, could we have 

 sharp lines of distinction between species. 

 Even in the case where the new form had 

 migrated to a region not occupied by the 

 old, the transitional forms would be dis- 

 closed on studying the species in aU its 

 range. Distinctions of species in such a 

 case must necessarily be more or less a con- 

 ventional matter. But if species originate 

 by the sudden production of entirely new 

 characters, that is by mutations, as de 

 Vries believes, then there are no transi- 

 tional forms connecting the new to the old. 

 The condition in nature in this case would 

 be similar to that in which there has been 

 an extinction of the transitional forms be- 

 tween two different types derived from a 

 common ancestor by gradual modifications. 

 In either case there is room for individual 

 judgment in the delimitation of species, 

 according as the differences between the 

 two types are greater or smaller. We say, 

 "A species is a perennial succession of like 

 individuals." But how nearly alike must 

 they be? 'No two individuals are exactly 

 alike, and the extreme differences possible 

 between two individuals of the same species 



may be greater than those between two 

 individuals of different species. In other 

 words, the differences within the species 

 may be greater than the differences be- 

 tween species, as de Vries has pointed out. 

 How then are we to decide whether two 

 individuals comparatively different from 

 each other, and yet alike, belong to the 

 same or different species? It has been 

 found that for any given character the 

 variations within the species may be ex- 

 pressed numerically by an average with 

 deviations, both above and below that av- 

 erage. For instance, the average height 

 of the stem in a given species of plants 

 may be two feet. Most of the plants com- 

 posing the species may vary only slightly 

 from this average, say from one to three 

 feet. But the greater the number of indi- 

 viduals examined and measured the more 

 certain it becomes that we shall find a few 

 individuals which differ far more widely 

 from the average. In our supposed case 

 we might find that the extreme limits of 

 size were six inches to ten feet, while the 

 average was only two feet. These devia- 

 tions from the average of the species are 

 called the fluctuating variations. They are 

 largely determined by the external condi- 

 tions in which the species grows. Pro- 

 fessor George Klebs has shown that when 

 plants are subjected to extremes of varia- 

 tion in the external conditions of light, 

 heat, moisture and food supply, the devia- 

 tions from the average of the fluctuating 

 variations become far greater than are usu- 

 ally found in a state of nature. Klebs 's 

 results with Sempervivum were truly re- 

 markable. He produced variations that 

 are not found in a state of nature in the 

 species with which he worked, changes in 

 the color, size and shape of the flower, 

 great variations in length of the stem and 

 its mode of branching, the size, shape and 

 arrangement of the leaves. As the result 



