of development must either be parallel, divergent or convergent. 
Convergency of the lines must especially often obtain in those cases 
where evolution tends towards simplification. Examples : A longi- 
tudinally striped species and a transversely striped one are very 
different, ‘but the pattern becomes similar in both when the 
stripes break up into spots. Originally more complicated patterns 
are frequently so much simplified that the Insect is almost uniform 
in colour, for instance all white or all black. 
Now, if it were true that the line of development followed by a 
species only depended on the constitution of the Insect, a species 
could not split up into several divergently developing species. But 
apart from that, the hypothesis does not take account of many 
very striking facts of resemblance. It does not explain, for instance, 
why the American « mimetic » Papilios of the /ysithous-group 
(distantly related to Papilio podalirius) are similar to American 
Aristolochia-Papilios,while the corresponding Old World swallow- 
tails resemble Danaids; and why generally the « mimicking » 
species are similar to « models » which occur in the same faunistic 
district. This difficulty is overcome by the theory that, thirdly, 
evolution stands under the direct influence of the physical environ- 
ment. Development, it is said, can be accelerated, retarded, 
stopped, reversed, and thrown out of its direction by the sur- 
roundings acting directly on the constitution of the species, e. g. 
climate, food, and physical and chemical properties of the soil. 
« Like cause like effect » explains the similarity in non-related — 
species. Some of the above examples might be considered as evid- 
ence for the correctness of this theory. We mention further the 
numerous modifications which appear under the influence of the 
seasons, the frequently minute variation according to locality, the 
pale colour of cave Insects, the results of temperature experiments, 
and the often great similarity in the structure of parasites (e. g., 
Pulicida, Nycteribia, Polyctenes, Platypsvlla). 
Lastly, an explanation of the similarities we have under consi- 
deration is offered by the theory of Mimicry. Although the 
surroundings are also in this case the agents which bring about the 
resemblance, the theory stands nevertheless in sharp contrast to 
the one just mentioned. The similarity between the mimic and the 
model is the result of natural selection (by enemies, such as Birds 
and Lizards) having weeded out the less similar specimens, which 
tended to render the species more and more similar to its model 
