Why and how the external effects are limited to special details of the
structure we do not know; but it does not seem as if any far-reaching
conclusions as to the cumulative effect of external conditions on the
higher terrestrial animals and plants, can be drawn from such an
exceptional phenomenon. It seems rather analogous to those effects of
external influences on the very lowest organisms in which the vegetative
and reproductive organs are hardly differentiated, in which case such
effects are doubtless inherited.[208]

[Illustration: FIG. 36. _a._ Branchipus stagnalis. _b._ Artemia salina.]

_Professor Geddes's Theory of Variation in Plants._

In a paper read before the Edinburgh Botanical Society in 1886 Mr.
Patrick Geddes laid down the outlines of a fundamental theory of plant
variation, which he has further extended in the article "Variation and
Selection" in the _Encydopaedia Britannica_, and in a paper read before
the Linnaean Society but not yet published.

A theory of variation should deal alike with the origin of specific
distinctions and with those vaster differences which characterise the
larger groups, and he thinks it should answer such questions as--How an
axis comes to be arrested to form a flower? how the various forms of
inflorescence were evolved? how did perigynous or epigynous flowers
arise from hypogynous flowers? and many others equally fundamental.
Natural selection acting upon numerous accidental variations will not,
he urges, account for such general facts as these, which must depend on
some constant law of variation. This law he believes to be the
well-known antagonism of vegetative and reproductive growth acting
throughout the whole course of plant development; and he uses it to
explain many of the most characteristic features of the structure of
flowers and fruits.

Commencing with the origin of the flower, which all botanists agree in
regarding as a shortened branch, he explains this shortening as an
inevitable physiological fact, since the cost of the development of the
reproductive elements is so great as necessarily to check vegetative
growth. In the same manner the shortening of the inflorescence from
raceme to spike or umbel, and thence to the capitulum or dense
flower-head of the composite plants is brought about. This shortening,
carried still further, produces the flattened leaf-like receptacle of
Dorstenia, and further still the deeply hollowed fruity receptacle of
the fig.