THE YOUNG NATURALIST, 



209 



VARIATION IN LEPIDOPTERA. 



INTERMEDIATE FORMS. 



By JOHN E. ROBSON. 



Specimens presenting a marked departure from the normal type, have 

 always been much valued by collectors, and since Darwin showed the im- 

 portance of the subject, various explanations have been given more or less 

 satisfactory to account for some of the phenomena. Mimicry of other 

 species, assimilation in colour and appearance to the hues of the habitat or 

 resting place, are among the more important suggestions. Lord Walsingham 

 has recently, in a very important paper of which a lengthened notice will 

 subsequently be given, argues with great force and clearness, that northern 

 and mountain species of Lepidoptera are dark, because dark colours absorb 

 heat more rapidly, and they thus mature more quickly ; an important ele- 

 ment in places where summers are shorter and the colds of winter prevail for 

 a longer period, than in sheltered valleys or southern climes. 



While these theories explain much of the change that appears to have 

 taken place in years gone by, and that is still in progress, much remains to be 

 explained, and without having any explanation to offer, I wish to direct atten- 

 tion to an important point in the investigation : — The general absence of Inter- 

 mediate Forms, even of those species of which there are the largest number 

 of permanent varieties. 



That " like produces like " may be taken as a law lying at the very root of 

 the cause that produces permanence of species. Yet it is equally true that 

 like never produces like. No two specimens of anything, animal or vegetable, 

 are ever so exactly the same as not to be individually recognisable by any 

 one accustomed to their appearance. The lepidopterist looking down his 

 rows of insects, can tell you where every specimen came from, recognising 

 the difference betweeen each. Yet he will look at a collection of coleoptera 

 and think them all alike, being less accustomed to their appearance. The 

 differences between individuals may be minute, and only the educated eye 

 be able to observe it, but it is there always. 



Besides these "individual differences" there are many others requiring 

 consideration. The sexes, even in animals so low in the scale as insects, differ 

 in appearance as well as in structure. The same species will vary according 

 to locality, being darker or paler, larger or smaller, or changing in other 

 ways. Some species have produced very distinct varieties, differing from the 

 type in a marked manner, yet occurring with it, not perhaps in every place, 

 but only in some. Others have produced equally well marked forms, that do 

 not occur with the type, but have become the only form where they are found. 



