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in another insect? Or why should these two species be thus 
differentiated from all their congeners, without any peculiarity in 
their habits, that we can discover, tc give occasion for the 
anomaly? We can return no answer to these questions. Final 
causes utterly fail us here. Nor can science, even looking to its 
most recent achievements, do more than partially unravel the 
mystery. It would refer us to Darwin, in whose works the 
whole subject of variations of structure, in animals as well as 
plants, has been more thoroughly considered and gone into than 
it ever had been before. But Darwin himself hardly ventures to 
affirm what it is that causes structure to vary in the first instance. 
He only deals with the fact in its consequences. And he is led 
to believe that, however slight the variation may be at first, and 
from whatever cause it may arise, if it be of any use to the 
animal in which it occurs, or give the latter any advantage 
beyond what it had before, the variation will be availed of and, 
from use, go on increasing, along with, it may be, more or less of 
a corresponding change in the animal’s habits. And these two 
variations—one of structure the other of habits—will continue, 
acting reciprocally on each other, so long as the environment 
remains the same. Probably, however, not longer; for what 
may be an advantage under some circumstances might be a 
disadvantage under other circumstances. In this way then 
structures may grow and improve, or deteriorate and finally waste 
away altogether, according as the conditions to which they are 
subjected are favourable or otherwise. Further; this growth 
or decay of organs is influenced by another circumstance, much 
_ dwelt upon by Darwin, viz., that whatever of change is brought 
about in one generation is inherited, for good or for evil, by the 
~ generation that follows after. 
It may be asked now how all this applies to the case of the 
“insect before us? It applies so far as to suggest under what 
_ circumstances, and by what means, its antenne may have attained 
to their present length. Some slight increase of development, 
