1897. SOME NEW BOOKS. 335 



A Study in Variation. 



On Mechanical Selection and other Problems. By Karl Jordan. Pp., no., 

 iv. plates. (From Novitates ZoologiCig, vol. iii. Tring ; 1896.) 



This memoir is the record of a careful and valuable piece of research 

 which throws light on some factors of organic evolution. Dr. Jordan 

 has compared the variation of the genital armature in a number of 

 species and sub-species of butterflies {Papilio, auctt.), with the varia- 

 tion of the wing-patterns, and has been able to formulate several 

 suggestive generaUsations from the facts established. Incidentally he 

 discusses many current problems. 



In his introductory remarks, the author ventures to answer a 

 question more easy to ask than to solve. " What is meant by a 

 species ? " He criticises the definitions of the term which have been 

 given by Eimer, Romanes, and Wallace, and concludes that " the 

 chief criterion of specific distinctness of a given form of animal or 

 plant is the impossibility of fusion with other forms." There comes 

 a point, according to Dr. Jordan, in the development of races, when, 

 a certain degree of divergency having been attained, " the form 

 exhibiting this divergency can never become one with any other 

 form." AH distinctions short of this point are treated as varietal or 

 sub-specific, and these lesser divisions are held by the author to differ 

 from the true species, not only in degree but in kind. Even granting 

 that there is a point of divergence past which fusion becomes 

 impossible, doubt as to whether certain forms have or have not gone 

 beyond it must always remain until set at rest by experiment ; so 

 that there will be room for differences of opinion among those who 

 accept Dr. Jordan's conception of the species. But not every 

 naturalist will agree with his premise that the course of evolution 

 must be represented by a tree with constantly dichotomous branching. 

 " If we should have to concede the possibility," he writes, " that the 

 lines of ancestors of any two forms of plants and animals, say of the 

 lion and the giraffe, were such that they first became widely divergent, 

 then identical, then again widely divergent, not only the figurative 

 tree, but also the kind of evolution it is meant to illustrate, would be 

 pure nonsense." The lion and giraffe are rather extreme examples, 

 but the opinion that lines of descent might converge or anastomose as 

 well as diverge would not be summarily set down as "pure nonsense" 

 by all who have thought on the subject.' 



Dr. Jordan adopts the trinomial form in naming the sub-species 

 with which he deals, and in cases where the name first given to an 

 individual of the species is evidently applicable to a particular sub- 

 species he holds that that name must be repeated to distinguish that 

 subspecies, e.g., Papilio priamns priamus. Sometimes indeed the 

 specific name has been applied to a particular form of one sex in a 

 polymorphic sub-species; then we get Papilio polytes polytes ? .-{. polytes. 

 " The meaning of this name is exclusive and hence precise, and that 

 is the highest praise we can give to a name." It will be re-assuring 

 to those not enthusiastic over such results of advanced nomenclature, 

 to know that not many butterflies have to be labelled after that 

 fashion. 



The parts whose variation is particularly described are the haypes, 

 or series of ridges, teeth, and hooks to be found on the inner surfaces 

 of the paired valves or claspers, which are lateral wings of the ninth 

 abdominal segment in male butterflies. Detailed descriptions and 



1 Cf. E. W. McBride, Nat. Sci., vol. x.. p. 31. Jan., 1897. 



