RECENT LITERATURE. 103 



for descriptive purposes must surely lay them aside when they look 

 at the figures in this volume, where, in the three hundred odd figures, 

 it would be impossible to find one that could be confused with any 

 other. Mr. Pierce says he sees no difference between favicolor and 

 pallens except size, yet if his figures can be relied on there is consider- 

 able difference in some of the proportions and in the spinous 

 armature of the cucullus. It is a case like this in which a photo- 

 graph, even a poor and imperfect one, would much exceed in value 

 any drawing. There are, however, two circumstances which make 

 the appendages rather more valuable than any other structure, or 

 even than a combination of other characters, for the decision of a 

 question either of specific distinction or of generic or familiar 

 alliance. One of these is that so long as two forms remain one 

 species, that is, are syngamic, or breed together, directly or in- 

 directly, these parts are kept identical in both. As soon as they 

 become distinct species, divergence of the parts is liable to occur, 

 perhaps rapidly. Only syngamy can keep these structures uniform 

 throughout a group of forms. When syngamy ceases, they are free 

 to vary, the only necessity being that they remain identical through- 

 out each syngamic group. But their variation is not restrained by 

 any such question as adherence to a food-plant, avoiding particular 

 enemies, &c., as nearly all other characters used to define species are 

 characters that are kept constant by natural selection. The other 

 circumstance is that they are highly organised and for the most part 

 hard, chitinised structures, so that they afford many details for 

 observation and these details are embodied in forms less difficult to 

 seize than many such items as colour, wing markings, &c. 



It is now well known. Dr. Jordan having perhaps most clearly 

 pointed the fact out, that geographical races have these appendages 

 varying a little in each race. In some cases, when the segregation 

 is great, these no doubt mark incipient species ; when the segregation 

 is incomplete, as at the extremities of an extended habitat, there may 

 be a difference that is prevented from becoming great by mediate 

 syngamy. 



When there is immediate syngamy throughout a group of forms 

 (unquestionable species) we may expect to find great uniformity 

 throughout the group, but it is highly probable that in such a group 

 variations may take place very rapidly, the whole group moving 

 together. This seems a conclusion inevitable from the many instances 

 we meet with in which the appendages are extremely different in 

 closely allied species. 



Unquestionably the appendages, like everything else, are under the 

 control of natural selection, but as to what the circumstances are 

 that tend to govern the selection, we are certainly at present very 

 much in the dark, so that if our argument seems to suggest that 

 they have escaped its control, all that is postulated is that they are 

 not controlled by any of those items of environment that have to be 

 counted on by most other characters, and that as a result two 

 newly established species may retain by force of similar environment 

 a very similar facis and structure in everything but these structures, 

 which are free to vary to any extent if only they vary together 

 throughout each specific group. 



