bates’ insect eauna. 
r 
L.. 
223 
and force. He shows that some of the forms of Leptalis , whether 
these he ranked as species or varieties, which mimick so many other 
butterflies, vary much. In one district several varieties (which are 
figured) occur ; one alone of these pretty closely resembles the com¬ 
mon Ithomia of the same district. In a few other cases, this Leptalis 
presents two or three varieties, one of which is much commoner 
than the others, and this alone mocks an Ithomia. In several cases 
a single Leptalis , which sometines must be ranked, according to the 
usual rules followed by naturalists, as a variety and sometimes as a 
distinct species, mocks the common Ithomia of the district. Prom 
such facts as these, Mr. Bates concludes that in every case the 
Leptalis originally varied; and that when a variety arose which hap¬ 
pened to resemble any common butterfly inhabiting the same district 
(whether or no that butterfly be a variety or a so-called distinct species) 
then that this one variety of the Leptalis had from its resemblance 
to a flourishing and little persecuted kind a better chance of escaping 
destruction from predacious birds and insects, and was consequently 
oftener preserved ;—“ the less perfect degrees of resemblance being 
generation after generation eliminated, and only the others left to 
propagate their kind.” This is Natural Selection. Mr. Bates 
extends this view, supporting it by many facts and forcible arguments, 
to all the many wonderful cases of mimickry described by him. He 
adds, “ thus, although we are unable to watch the process of forma¬ 
tion of a new race as it occurs in time, we can see it, as it were, at 
one glance, by tracing the changes a species is simultaneously under¬ 
going in different parts of the area of its distribution.” 
To the naturalist who is interested with respect to the origin of 
species, the most important parts of this Memoir, together with the 
descriptive portion at the end, are probably those which treat on the 
limits of species, on sexual variation, on the variation of important 
characters, such as the neuration of the wings, &c. We cannot here 
discuss these points. Mr. Bates shows that there is a perfect grada¬ 
tion in variability, from butterflies, of which hardly two can be found 
alike, to slight varieties, to well-marked races, to races which can 
hardly be distinguished from species, to true and good species. Un¬ 
der this point of view, the history of Mechanitis polymnia well de¬ 
serves study: after describing its several varieties, Mr. Bates adds, 
“ these facts seem to teach us that, in this and similar cases, a new 
species originates in a local variety, formed in a certain area, where 
the conditions are more favourable to it than to the typical form, 
and that a large. number of such are simultaneously in process of 
formation from one variable and widely distributed species.” It is 
hardly an exaggeration to say, that whilst reading and reflecting on 
the various facts given in this Memoir, we feel to be as near wit¬ 
nesses, as we can ever hope to be, of the creation of a new species 
on this earth. 
We will only notice briefly one other point which has an impor¬ 
tant bearing on the production of new races and species; namely the 
