SOME BIOLOGICAL PROBLEMS.
11
which carry both male and female factors although showing
only female characters and which produce, presumably without
a reduction division, only "summer" eggs not requiring
fertilisation; (2) females which produce "winter" eggs
requiring fertilisation and therefore presumably following a
reduction division; (3) females which produce at different
times both "summer" eggs and "winter" eggs; and (4) males.
It would be very interesting to know exactly what does take
place in the cell divisions leading to the production of these
differently constituted forms, but I believe no observations
have been made to elucidate the matter. And even if the
mechanism of the determination of sex were known there would
still remain the third point of interest, namely, the cause of
the production of the particular forms at the particular times
when they do appear. A good deal of work has been done on
this subject, but even now the answer is by no means quite
definite. That in a general way seasonal conditions govern the
length of the period of parthenogenetic reproduction and the
appearance of males and "ephippial" females is evident, but
there is so much variation in particular cases that the factors 
directly responsible are probably numerous and only in a rather
roundabout way dependent upon the seasons.
Another physiological problem which merits consideration
is that of the production of colour and its frequent arrangement
in definite patterns. Colour, whether pigmental or structural,
is almost certainly in its origin merely a by-product of the
metabolism of the organism under various conditions, without
in the first instance having any significance from an adaptational
point of view. But, as in so many other cases, colour can be
turned to good account in many ways, e.g., for concealment,
for sexual attraction, for recognition, for warning, etc., and
has been so made use of by perhaps the majority of animals.
These uses, however, usually invoke not only the production of
colour, but its arrangement in characteristic patterns and the
explanation of this is still very obscure. In the higher types
of both vertebrate and invertebrate animals it is usually quite
impossible to give any satisfactory physiological reason why
certain patterns should be produced rather than others. Even
in the lower types it is more often than not very difficult to
understand the arrangement of the colours, but at any rate