150 The Philippine Journal of Science 1916 



familiar structures is understood at the present time. Addi- 

 tional structures are being interpreted in terms of utility from 

 time to time. I can remember when the serrate leaf-margin 

 was apparently a good example of a distinctive structure pre- 

 sumed to be beyond the reach of selection; but, for the past 

 decade, the serrate margin has been understood to be useful 

 under particular conditions and to be correlated in usefulness 

 with other adaptive structures. 



Doctor Willis continues to preach mutation in his latest paper, 

 and I would argue at greater length against the application to 

 natural selection of his opinions on mutation,* if it were not 

 that he has himself presented with striking clearness a con- 

 clusion which does away with any necessity of argument on 

 the subject. In the Philosophical Transactions (pp. 329, 330) 

 he says : 



We have no criterion to go by, by which to affirm that a certain specific 

 difference is "small" and another "large." We have no right to say, 

 for example, that if a leaf of one species is simple and of another compound, 

 this is a larger difference than if one is pinnatifid, the other pinnatipartite. 

 We have not the least idea whether the changes in internal construction 

 of the nucleus necessary to form Jordanian species are in any way different 

 from, or smaller or larger than those necessary to give Linnaean species. 



And a little farther along, 



We must simply take account of all definite and hereditary differences, 

 whether we consider them large or small! Every one appears to imply a 

 mutation, but whether some mutations are large and others small, we 

 have no idea, for we do not know in what a mutation really consists. 



It seems to me better to adopt the hypothesis that any specific difference 

 may appear at one step, whether it be large or small. But we may go 

 further than this, and claim that even "larger" differences than any we 

 have as yet discussed may also arise at one step. For instance, the 

 endemic Coleus elongatus on the top of Ritigala differs so much from all 

 other Colei in its equally toothed calyx, and raceme-like inflorescence, as 

 well as in other points, that it must probably be regarded as almost, if 

 not quite sub-generically distinct. Yet the whole species is confined to the 

 summit of this one mountain and exists there as about a dozen individuals, a 

 number which can never have been much exceeded, if at all; and it must 

 in all reasonable probability have arisen there at one step. 



But even with the formation of a sub-genus the possibilities of single 

 mutations do not cease. * * * The distinction between genus and species 

 is really more or less artificial, depending upon our ideas as to what are 

 large and what are small changes. 



With these ideas, I am in most complete accord. As long ago 

 as 1904, 2 I concluded (p. 426) that "Mutations, or discontinuous 

 variations, and the most insignificant of individual variations 



The variations of some California plants. Bot. Gaz. 38 (1904) 401-426. 



