286 STUDIES IN THE EVOLUTION OF ANIMALS 



and the beginning of the Trias, in the midst of this transition 

 of physical disturbance, appear the great reptilian forms char- 

 acteristic of the age of reptiles, and the earliest precursors of the 

 mammals, and at the same time the old carboniferous forms of 

 plants finally pass away to be replaced by a flora scarcely more 

 advanced, though different, and consisting of pines, cycads, and 

 ferns, with gigantic Equiseti.' 



A little exercise of the imagination will enable us to compre- 

 hend that in those volcanic periods, both the sea and the atmo- 

 sphere may have had gases and other ingredients in them which 

 might either kill, or strangely modify, the development of the ova 

 and seeds on which they acted. Heat, electrical changes, and 

 continual tremors of the earth and sea, may have also been factors 

 in the modification of both animal and vegetable embryos. We 

 know what strange modifications of buds and other parts of plants 

 are produced by even the tickling of the minute larva of a Gall-Fly. 



In our present comparatively quiet times, with a settled and 

 steady composition of atmosphere and sea, we may have difficulty 

 in conceiving and measuring the amount of modification that may 

 have occurred when the conditions must have been wholly different. 



We should not forget, however, that, although not generally 

 admitted, the action of changes in the environment in modifying 

 forms, already established elsewhere, is of importance.^ 



Some species no doubt are very plastic and can easily change 

 their form under changed conditions ; others do not, while there 

 are some that are killed outright. 



At a lecture of the Royal Horticultural Society, Mr. Burbidge 

 mentioned that Nepenthes Raja, from Borneo, cannot be grown 

 in this country under artificial conditions, even with the skill 

 and scientific manipulations of Messrs. Veitch and Sons. 



' See Nature, vol. 43, p. 581, for change of species in Arabis anachoretica. 



