884 



SCIENCE 



[N. S. Vol. XXV. No. 649 



of this kind of work, carried on for a con- 

 siderable number of years, Klebs has given 

 us a definition of a species which expresses 

 the dependence of the form of the plant 

 upon the environment. According to Klebs 

 we must say: "To a species belong all indi- 

 viduals which, propagated vegetatively or 

 by self-fertilization, under like external 

 conditions, show the same characters 

 through many generations." If two 

 plants under these conditions show a 

 noticeable difference, they are to be re- 

 garded as belonging to two species, even 

 though they have descended from a com- 

 mon ancestor. Gaston Bonnier has shown 

 by experiment that plants transplanted 

 from the region of Fontainebleau near 

 Paris to Toulon in the Mediterranean re- 

 gion show in a few years adaptations both 

 of external form and internal structure 

 which cause them to resemble the species 

 characteristic for the Mediterranean re- 

 gion. The same investigator found similar 

 results on transplanting from the plains to 

 Alpine regions. Knowing the origin of 

 such widely variant forms we do not call 

 them two species, but merely extremes in 

 the fluctuating variations of the species. 

 It is conceivable, however, that nature 

 might perform this same experiment on 

 such a scale and in such ways as to make 

 it difficult or impossible to recognize the 

 common origin of two such different types. 

 In that case the botanical collector or sys- 

 tematist, finding the two types in widely 

 separated regions, would describe them as 

 two species of plants. If the distribution 

 of the species was continuous from one of 

 these extreme regions to the other the con- 

 necting intermediate forms would show 

 that we had to do merely with extreme 

 fluctuating variations brought about by ex- 

 tremes in soil, moisture, heat and light. 

 If, however, the geographical continuity of 

 the species had been interrupted in any 



way, it would be impossible to determine 

 by observation alone that the two extreme 

 types were only fluctuating variations of 

 one species. That could be determined by 

 the experimental method as followed by 

 Klebs and Bonnier. Plant the two types 

 in the same region, grow them under ex- 

 actly the same conditions, and if after 

 many generations they continue to exhibit 

 constant differences they are to be regarded 

 as two species. On the contrary, if they 

 show the same characters under the same 

 conditions, they are one species. Such a 

 method of determining whether one has a 

 new species or not involves an enormous 

 amount of labor, and a great deal of time. 

 It is not in favor with the systematists 

 who work with the higher plants. Never- 

 theless, there is an increasing recognition 

 among botanists of the necessity of phys- 

 iological work even in those fields of re- 

 search that have in the past been .domi- 

 nated by morphology alone. 



Such experiments might help to decide 

 the question whether the so-called alpine 

 species have been constant since the glacial 

 period, as de Vries supposes they must 

 have been, or whether, as seems possible, 

 similar combinations of climatic conditions, 

 operating in widely separated regions such 

 as the alpine region of central Europe and 

 the high latitude of Norway, have pro- 

 duced species of similar form. It does 

 not even seem necessary to assume that the 

 parent species of the alpine forms has been 

 the same in these widely separated regions. 

 De Vries has pointed out that species some- 

 times overlap by what he calls transgres- 

 sion variations. Klebs has shown that in 

 one species of Sempervivum he could pro- 

 duce nearly all the characters found in 

 the other species of the genus. Is it not 

 therefore possible that the continuation of 

 conditions of soil, temperature, moisture 

 and light characteristic of the alpine re- 



