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 Equisetum, Lycopodium, Selaginella, Isoetes, 

 Marattia, Marsilia, and Pilularia 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, Lepidodendron is virtually cosmopolitan. Plants 

 of the Glossopteris 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." 1 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. 



