466 Evolution by Segregation. [August, 
less finds its advantage in so doing. This resemblance we 
can to some extent comprehend on the theory of Natural 
Seledtion. But we can scarcely admit that the bug has set 
out on its travels to search for some more powerful insert 
for which it might be mistaken, and having recognised such 
a one in the Priocnemis has taken up its abode in localities 
inhabited by the latter. Or, to take a somewhat similar 
case, the caterpillar of the lobster-moth ( Stauropus fagi) has 
in some of its attitudes a very decided resemblance to a 
spider, and might easily be mistaken for one when lurking 
among leaves. Ichneumons have for the most part a great 
dread of spiders, and it is hence exceedingly probable that 
such a resemblance may save these larvae from the attacks 
of their chief enemy. It is here exceedingly difficult to ex- 
plain this mimicry by emigration. In every country where 
the food-plant of the larva — the beech tree — grows, spiders 
are present. Whence, then, can the insert have come in 
search of a formidable something which it resembles ? The 
change of its food-plant would not affedt the question at all. 
Very many indeed of the mimetic features of inserts seem 
inexplicable upon the migration theory. 
As regards “ desert-forms ” of life there is also a difficulty. 
Canon Tristram observes that the fauna of Arabia is more 
thoroughly desert-coloured than that of the Sahara, and 
hence draws the very plausible conclusion that the Arabian 
deserts are the older, the fauna of the Sahara not having 
yet had sufficient time to become adapted to its surround- 
ings. This difference between the faunae of the two deserts 
seems to us in harmony rather with the hypothesis of Darwin 
than with that of Herr Wagner, — with the supposition of 
selection rather than with that of immigration. 
On attempting to summarise the considerations put for- 
ward above, it seems to us that Segregation is more likely to 
favour the establishment of new species than is Seledlion, 
which latter appears to be adtually reducing the number of 
existing forms. Segregation also can more readily take into 
account the influence of those external agencies which must, 
occasionally at least, exert a transforming power upon 
organic species. On the other hand, Herr Wagner’s ex- 
planation of the phenomena of mimetism seems very 
questionable. 
Can the two theories be reconciled ? We doubt it, al- 
though Prof. Semper is of opinion that Segregation is 
included in Darwinism. It must be admitted that the 
struggle for existence may often prove the cause which 
