44 
Satyrids, or of the instances of peculiar structure in the same sex among 
certain Agaristide) are hardly known to me as yet, a single preparation 
will commonly suffice. The variation in the neuration of a single species 
is usually very slight. Where it is perceptible it falls under two heads : 
(1) the tendency to form extra fragmentary veins, or again, the sudden 
obliteration of a part of a normal vein ; (2) a variation in the position 
of the veins which is found to fall in with the general direction taken 
by the neuration in evolution. ‘The first may be characterised as 
abnormal. Its cause is obscure to me; and while the sudden 
appearance of a stump of a new vein may be looked upon as a dis- 
play of negative growth, the disappearance of a part of a normal 
nervure or vein may be directly owing to failure in the supply of 
nutrition. But the zorma/ variation cited under (2) is by far the 
more interesting. By it I have been furnished with proof that the 
general direction and progress detected by me in the specialisation of 
the wing is correct. Thus the individuals of a single species would 
present different grades of specialisation, just as the assemblages of 
individuals which we characterise as species present different grades 
of the same specialisation when compared with each other. In- 
stances of the aduorma/ class of variation have occurred to me in the 
genera Melitea, Endromis, Smerinthus. From an examination of a 
series of Copismerinthus ocellata I have been able to establish various 
grades of zorma/ variation in the position of the branches of the 
media. 
But I am anticipating. The mounted photographs of all the 
forms proposed to be studied I arranged in lines on shelves, one 
after another, so that while walking before them I could compare the 
pictures, note the changes assumed by one and the same nervure in 
all of them, and alter their sequence. Up to this moment I had 
been generally content to note merely the differences in the location 
of the veins. In no paper or book accessible to me was there any 
indication of a plan or order of these differences having been 
observed. Always were the changes stated as a mere matter of 
description,—this vein was thrown off by that one, and so on. Even 
in the best work we have on the subject, that of Professor Comstock, 
there is no distinct recognition that in different parts of the wing 
there is a regularly indicated specialising direction and progression, 
and that there are here offered distinct gauges of the amount of 
specialisation exhibited in each of these special cases.* But now, as 
I made, day after day, these comparisons, the rigid network clearly 
showed a plastic movement in certain directions ; the evolution of the 
lepidopterous wing thus became clearer to me. And the principal 
nervure influencing, seemingly by mechanical means, these changes 
appeared to me to be the radius, so that a study of the radius was 
here pressed upon my attention. From its position in the wing the 
strength of the downward stroke would be naturally registered by the 
* My obligations to Comstock’s work are very great; no other source has 
been in a general way more helpful to me. 
