August 14, 1914] 



SCIENCE 



243 



with wliieh DeVries has now made us fa- 

 miliar by his writings on the mutation theory. 

 In so far as mutations may consist in meristic 

 changes of many kinds and in the loss of 

 factors, it is unnecessary to repeat that we 

 have abundant evidence of their frequent oc- 

 currence. That they may also more rarely 

 occur by the addition of a factor we are, I 

 think, compelled to believe, though as yet the 

 evidence is almost entirely circumstantial 

 rather than direct. The evidence for the oc- 

 currence of those mutations of higher order, 

 by which new species characterized by several 

 distinct features are created, is far less strong, 

 and, after the best study of the records which 

 I have been able to make, I find myself un- 

 convinced. The facts alleged appear capable 

 of other interpretations. 



" DeVries found, as is well known, that 

 (Enothera LamarcMana gives off plants un- 

 like itself. These mutational forms are of 

 several distinct and recognizable types which 

 recur, and several of them breed true from 

 their first appearance. The obvious difficulty, 

 which in my judgment should make us un- 

 willing at present to accept these occurrences 

 as proof of the genesis of new species by mu- 

 tation, is that we have as yet no certainty 

 that the appearance of the new forms is not 

 an effect of the recombination of factors, such 

 as is to be seen in so many generations of 

 plants derived from a cross involving many 

 genetic elements." 



The phenomenon of twin-hybrids he does 

 not consider satisfactory evidence of group 

 inheritance of characters, but to have its best 

 explanation " in the well ascertained fact that 

 the male and female germ-cells of the same 

 individual may be quite different." By this 

 is meant that the pollen and ovules of the 

 same plant may transmit different qualities 

 respectively. 



In order to throw light on the question 

 whether species originate discontinuously or 

 not, Bateson discusses, in successive chapters, 

 " Variation and Locality," " Overlapping 

 I'orms," and " Climatic Varieties," bringing 

 together a great amount of illustrative ma- 

 terial partly of his own collecting, partly the 



work of others. Special attention is given to 

 the nearly related species of North Ameri- 

 can flickers and of warblers, which are illus- 

 trated by two beautiful colored plates. These 

 cases have been selected because they seem to 

 show specific differences consisting in Men- 

 delizing unit characters. Even in these cases, 

 however, the existence of such unit characters 

 is inferential rather than demonstrated and 

 actual experimental work on the crossing of 

 wild species of birds, such as pheasants studied 

 by Ghigi and Phillips, and pigeons studied by 

 Whitman, though it reveals the frequent oc- 

 currence of unit character differences between 

 one domesticated variety and an original wild 

 species, rarely shows the existence of such 

 differences between one wild species and 

 another. Even granting that unit character 

 differences occasionally occur between one 

 wild species and another (as I believe they 

 do), it may well be that such differences, 

 though striking, are not the most important 

 or essential ones. As Bateson himself says 

 in another connection (p. 184), " It seems in 

 the highest degree unlikely that the outward 

 and perceptible character or characters which 

 we recognize as differentiating the race should 

 be the actual features which contribute ef- 

 fectively to that result." If an extensive sur- 

 vey were made of related wild species of birds 

 or mammals, I suspect that it would be found 

 that the discoverable differences in a majority 

 of cases consist in quantitative differences in 

 characters, rather than in presence and ab- 

 sences of striking single characters. Con- 

 sider for example the genus Mus. The black 

 rat, M. rattus, as the experiments of Morgan 

 and Bonhote have shown, is distinguished 

 from M. alexandrinus by a single unit char- 

 acter difference. The two cross freely and 

 Mendelize on crossing, but without producing 

 any new third form, so far as we know at 

 present. The color difference between them 

 is a very striking one, but it appears to be 

 the only existing difference. The one might 

 be described as a color variety of the other. 

 Compare now M. alexandrinus with M. 

 norvegicus. The two are very similar in ap- 

 pearance. Only quantitative differences in 



