sect, xii.] INTRODUCTION. G7 



found that members of Meristic series may vary simultaneously 

 and collectively — and this is one of the most important generaliza- 

 tions which result from the Study of Variation — yet it is also 

 true that in Variation single members of such series may vary 

 independently and behave as though they possessed an " in- 

 dividuality" of their own. If ever it shall be possible to form 

 a conception of the physical processes at work in the division 

 and reproduction of organisms, account must be taken of both of 

 these phenomena. 



I know no way in which the nature of Discontinuity in Varia- 

 tion and the position of intermediate forms may be so well illus- 

 trated as by the closely parallel phenomenon of Sex. In the case 

 of Sex in the higher animals we are familiar with the existence 

 of a race whose members are at least dimorphic, being formed 

 either upon one plan or upon the other, the two plans being in 

 ordinary experience alternative and mutually exclusive. Between 

 these two types, male and female, there are nevertheless found 

 intermediate forms, "hermaphrodites," occurring in the higher 

 animals at least, as great rarities. Now though these inter- 

 mediate forms perhaps exist in gradations sufficiently fine to 

 supply all the steps between male and female, it cannot be 

 supposed that the one sex has been derived from the other, and 

 still less that the various stages of hermaphroditism have been 

 passed through in such Descent. Besides this, even though there 

 is an accurate correspondence or homology between the several 

 organs which are modified upon the one plan in the male and 

 upon another in the female, and though this homology is such 

 as to suggest, were we comparing two species, that the one had 

 been formed from the other, part by part, yet by the nature of 

 the case such a view is here inadmissible: for firstly it is im- 

 possible to suppose that either sex has at any time had the organs 

 of the other in their completeness, and secondly it is clear that 

 any hypothetical common form, by modification of which both 

 may have arisen, must have been indefinitely remote and could 

 certainly not have possessed secondary sexual organs bearing any 

 resemblance to those now seen in the higher forms. All this 

 has often been put, but the application of it to Variation is of 

 considerable value. For in the case of Sex there is an instance 

 of the existence of two normals and of many forms intermediate 

 between them, occurring in a way which precludes the supposition 

 that the intermediates represent stages that have ever occurred 

 in the history of the two forms. 



In yet another way Sex supplies a parallel to Variation. As 

 we know, the sexes are discontinuous and occur commonly in their 

 total or perfect forms. Now just as members of a Meristic seri« - 

 may present total variations independently of each other, so may 

 single members of such a series present opposite secondary sexual 

 characters, which may nevertheless be in each case complete. 



