634 Tta Distribution of Colour [November, 
expression for transversely banded), and between these types 
there are the most manifold transition-forms. Not unfre- 
quently all traces of pattern or design disappear at an earlier 
or later stage, leaving the animal concolorous. But even to 
the present day the species with higher stages of design 
pass through the lower stages in the course of their indivi- 
dual development. Thus tigrated species are in their youth 
spotted, and at a still earlier stage long-striped. Certain 
concolorous species— e.g., the lion — are in youth tigrated, 
earlier spotted, and earlier still long-striped. Hence Eimer 
concludes that in primitive ages there existed only striped 
forms, from which other designs have gradually arisen. We 
have thus a new confirmation of the biogenetic law, accord- 
ing to which the history of the individual is an epitome of 
the career of its ancestry. 
In these successive transformations Eimer establishes the 
fadt that everywhere the female sex retains a more juvenile 
colouration than the male, thus remaining on a lower stage 
of development. Hence the law of “ male preponderance. ” 
Eimer shows further— -and this is a most interesting fadt — 
that the transformations of design take place in a certain 
diredtion along the body. In lizards this occurs from 
behind forwards, so that every new property appears first at 
the posterior extremity of the body, extending gradually for- 
wards, whilst the original design remains longest in the 
anterior parts. This rule Eimer calls the “ law of undu- 
latory development,” and he traces it both in the history of 
the species and of the individual. 
Turning from reptiles to birds, mammals, Lepidoptera 
and their larvae, Herr Eimer traces the same three funda- 
mental types of design, and the same relations. In birds, 
especially, the biogenetic law of colouration and the law of 
male preponderance are very strikingly manifest. 
Young birds of allied genera or species have the same 
designs and the same colours, even when very distindt at 
maturity. The females generally retain more or less the 
original, youthful colouration, whilst the males of the 
species deviate most widely from each other in their adult 
stage. We may take as an instance such groups as the 
thrushes, or the Raptores. In these, and in many other 
cases, it may be observed that the plumage of the young, 
and of the mature hen, displays streaky spots following the 
diredtion of the length of the body. In the adult male the 
spots are diredted transversely, or disappear altogether, this 
change being accompanied by a variation in colour* 
These relations appear especially striking in birds of 
