6 SCOPE OF COMPARATIVE MORPHOLOGY 
earth’s surface. This is, however, applicable only within certain limits: 
one of those limits is imposed by the wide distribution of germs 
which is so prevalent in plants. Wherever the mechanism for dispersion 
of germs is highly elaborated, and successful, the traces of evolu- 
tionary history, as shown by geographical distribution, are apt to be 
obliterated. The consequence is that in practice such distribution is 
only available as evidence of descent within restricted limits. The 
great geographical barriers, such as the tropics, the greater oceans, and 
the more continuous mountain ranges, it is true, delimit at present certain 
areas of vegetation, within which evidence of value as contributory to 
a knowledge of descent may be gathered; but at best this applies only 
to the later phases of evolution, and geographical distribution of plants 
at the present day gives little clue, or perhaps none at all, to the origin 
of the great groups which constitute the Vegetable kingdom at large. 
The fact that such genera as Eyudsetum, Lycopodium, Selaginella, Isoetes, 
Marattia, Marsilia, and Piludaria are, within their several limits of 
temperature, virtually cosmopolitan shows how little can be expected from 
geographical distribution of living forms as a key to the evolution of 
early types. Among fossils, ZLepzdodendron is virtually cosmopolitan. Plants 
of the Glossoprerts flora, long thought to be distinctively southern, have 
recently been recognised from Russia. Such examples suggest that neither 
does the geographical distribution of fossils as yet give any certain 
evidence as to descent of the main phyletic lines. 
Another closely related branch of Botanical science is the study of 
organisms from the aspect of function and circumstance, as tested by 
physiological experiment. The intimate connection between form and 
environment is too obvious to need insistence here; but though the 
individual shows a high degree of plasticity under varying conditions, still 
there is a large field, embracing the very fundamentals of plant-form, 
such as the evolutionary origin of leaves, of roots, or of sporangia, which 
lies as yet outside the region of physiological experiment. Thus, however 
interesting the branch of physiological morphology may be, its scope is still 
narrowly limited. The method of experiment, with a view to ascertaining 
the effect of external agencies in determining form, is now nascent, and 
carries with it high possibilities. But it is well in the enthusiasm of the 
moment to keep in view the limitations which must always hedge it 
round. It is to be remembered that the effect of external conditions 
upon form is always subject to hereditary control, and that thus a large 
field is left open still for speculation. ‘This seems to have been forgotten 
by a recent writer, who remarks that “the future lies with experimental 
Morphology, not with speculative Morphology, which is already more than 
full blown.”! Though we may question the cogency of this antithesis, still 
the assertion contains an important truth, inasmuch as it accords prominence 
to experiment; but the case is overstated. All who follow the development 
1 Flora, 1903, p. 500. 
