ME. G. J. ROMANES ON PHTSIOLOGICAIi SELECTION. 411 



we shall have long to wait before we derive much light upon it. 

 But it is enough for the explanation which is furnished by Mr. 

 Darwin's theory of the evolution of adaptive structures by natural 

 selection, that the variations in question take place ; and similarly 

 as to the present theory of the evolution of species by physio- 

 logical selection. 



Whatever, therefore, may be thought as to the truth of this 

 theory, or as to the extent of its applicability, it is certainly 

 something very much more than a bare re-statement of fact. If 

 the evidence which I have presented on these points is accepted 

 (as it must be by the criticism with which I am dealing), the 

 explanatory value of the theory may be estimated by the con- 

 sideration that what Mr. Darwin has called the " mystery of 

 mysteries " * ceases to be mysterious in any other sense or 

 degree than the general fact that oflFspring do not always and 

 in every respect resemble their parents. The birth of a new 

 species becomes, for instance, less mysterious than the birth of 

 a child with six toes, inasmuch as the variation which it implies 

 is one of less departure from the specific type. Nay, it becomes 

 even less mysterious than the occurrence of what I have termed 

 individual incompatibility — a variation which, on account of its 

 apparently trivial character, Mr. Darwin apologizes for so much 

 as mentioning. Hence, unless it be denied that the clearing up 

 of a mystery constitutes an explanation, the present theory is 

 unquestionably an explanation of the only phenomena with which 

 it is concerned. Although it makes no attempt at explaining 

 the physiological causes which underlie the phenomena of vari- 

 ation in general, if the evidence which has been given be accepted, 

 the theory does furnish a real explanation of the origin of species, 

 by proving that there is one particular variation which, so often 

 as it has taken place, must necessarily have constituted the 

 originating cause of a new specific form. 



* Viz. — the problem of the origin of species, which, as shown in the preceding 

 paper, his theory of natural selection serves only in small part to explain. 



