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Such a study, and such conclusions, are, so far as they go, 

 interesting and valuable; but one feels some sense of dis- 

 satisfaction at these results alone, and seeks to know not 

 only the broad reason of the change that has gradually taken 

 place, but also the details of the process and the conditions 

 under which it first occurred. These details, comparable in 

 our analogy of the picture, to the mixing and laying on of 

 the colours, their blending and changing in the progress of 

 the work, one superseding another or mixing with it, changing 

 its whole tone — all these may be traced in such forms as we 

 call varietal, indicating thereby their inconstancy and sus- 

 ceptibility to the influence of new conditions of existence. 



Generic and specific differences are nearly always, if 

 traceable to their cause, found to have been or to be in some 

 way useful to their possessors ; but as regards varietal 

 characters, I think it not unlikely that we shall, when we 

 have a sufficiency of facts before us, be able to separate 

 them into two broad classes, the progressive and the re- 

 trogressive. I think this distinction will have to be very 

 carefully made, and that once decided in any case it will be 

 all-important in the history of the form, since it will simply 

 mean whether it is the commencement of a new and vigorous 

 race, more favoured than its ancestors, or a sign that the 

 species cannot surmount the difficulties of the new influences, 

 and will lose vigour or succumb. 



Having said so much, I will now try to put before you a 

 few of the special kinds of variation that have most interested 

 me, and endeavour, where I can, to point out their causes. 

 Roughly, for convenience sake, we may take them under 

 the heads of colour, form, size, and substance. 



First of all, colour- variation. Little or nothing being 

 known of the vast majority of animal pigments, it becomes 

 more especially difficult to ascertain the manner in which 

 these variations arise, or the immediate cause of their pro- 

 duction. Perhaps the simplest form of variation under this 

 head is albinism, or the lack of colour. Albinism, in some 

 form or other, appears to occur almost universally through- 

 out all classes of coloured organisms, and is doubtless in 



