186 



SCIENCE 



LN. S. Vol. XXV. No. 631 



the question is, whether this connecting fonn 

 represents a stage of development from one 

 extreme to the other, or, as the de Vries school 

 supposes, a hybrid between the extremes. This 

 question, in my opinion, may be settled in 

 many, if not in all, cases, by the facts of the 

 geographical distribution. If the overlapping 

 area of the ranges of the two extremes is occu- 

 pied by an intermediate form associated with 

 the two original forms, hybridization is indi- 

 cated; if, on the contrary, in the intermediate 

 area the intermediate form is exclusively rep- 

 resented, the latter very likely marks a con- 

 necting step in the development from one to 

 the other extreme. 



I am much tempted to illustrate this here 

 by an example I have discovered in the dis- 

 tribution of certain species of river crawfishes 

 (group of Oambarus propinquics) . But it 

 would take up too much space to give all the 

 facts. It will be presented to the public in 

 due time, in fact, the paper is just now going 

 through the press. Suffice it to say that both 

 of the above cases are illustrated in this ex- 

 ample, but that the final decision was possible 

 only after a thorough investigation of the 

 geographical distribution of these forms had 

 been made, by researches covering the whole 

 of the state of Pennsylvania, and parts of 

 Maryland, West Virginia and Ohio. 



MacDougal further says (p. 209) that " a 

 number of zoologists have assumed to speak 

 of the distribution of plants, with apparently 

 no basis except ' general information ' to the 

 effect that closely related species do not have 

 the same habitat. This has been variously 

 put, but the general meaning is as given." 

 On page 210 he talks of this as ' the idea 



mutation as a synonym of discontinuous varia- 

 tion. A mutation may be due to discontinuous 

 variation, and, as de Vries declares, generally is, 

 but not always. In this respect, MacDougal, in 

 the article referred to, certainly goes beyond de 

 Vries. If this is borne in mind, it is clearly seen 

 that Merriam's objection does not affect at all de 

 Vries's theory as a whole, but only the part of it 

 that says that discontinuous variation or salta- 

 tion is a necessary or frequent attribute of muta- 

 tion (in the sense of creating the faaulty to be- 

 come a true breeding form ) . 



* * * that closely related species do not occupy 

 the same region.' I believe I am included in 

 this number of zoologists, although I never 

 expressed such an idea; but since I do not 

 know of any other zoologist who did, I think 

 MacDougal refers to something similar I have 

 said. I expressed it thus :' ' two closely allied 

 species never occupy absolutely the same 

 range under identical ecological conditions.' 

 This, however, does not exclude the possibility 

 that parts of their different ranges may over- 

 lap, and that in certain regions they may be 

 found together, and, moreover, a case in point 

 has been described by me.° Thus it is evident 

 that MacDougal has misunderstood me, and 

 that he battles against a fancied idea, which, 

 indeed, is disproved by the instance given 

 (Opuntia fulgida and mammillata) , and to 

 which I am able to add numerous other ex- 

 amples of plants" as well as animals. But 

 this does not influence my contention, that 

 closely allied species of plants or animals 

 never possess precisely the same geographical 

 range. 



With reference to Jordan's opinion that 

 QSnothera lamarchiana might be a hybrid, 

 which is also held by the present writer, Mac- 

 Dougal thinks that it possibly might be a 

 good, natural species. This question, how- 

 ever, is not essential for the interpretation of 

 de Vries's experiments. The suggestion that 

 it might be a hybrid, or the fact that, in 

 Europe, it is an escaped garden-form, is ad- 

 vanced only to explain the remarkable vari- 

 ability of it. De Vries, assuming that it is as 

 good as a natural species, tries to account for 

 its wonderful capacity to throw off mutants, 

 which is not generally found among natural 

 species, by believing that there may exist, in 

 any species, a time or period of especially 

 vigorous and frequent mutation. But eon- 



' Science, June 22, 1906, p. 949. 



" Science, March 30, 1906, p. 504. 



• For instance, Orchis ustulata and tridentata, 

 two closely allied species, but of quite different 

 aspect, grow side by side (forming hybrids) upon 

 the meadows of the valley of the river Saale, near 

 Jena, Germany; on a certain hillside near Jena, 

 Oplirys muscifera and aranifera grow together 

 (also forming a hybrid). 



