456 PRIMARY FACl^ORS OF ORGANIC EVOLUI'ION. 



surface, and illumined by reflected light on the lower 

 surface, then the antheridia and archegonia will be 

 produced on the upper surface. Galls are produced 

 under the stimulus of the insect almost anywhere on 

 the surface of the plant. Yet in most cases these galls, 

 in a sense grown at 'random on the surface of a plant, 

 when placed in damp earth will give rise to a young 

 plant. In the hydroid, Tubularia mesembryanthemum, 

 when the polyp-heads are cut off, new heads arise. 

 But if both head and root be cut off, and the upper end 

 be inserted in the mud, then from the original upper 

 end not head-polyps, but root-filaments, will arise, 

 while from the original lower end, not root-filaments, 

 but head-polyps will grow. In Ciona intestinalis, round 

 a slit cut into the body- wall, a tubular process grew out, 

 forming a new mouth, while around the base of this, a 

 series of eye-spots, corresponding to the eye-spots 

 round the real mouth, appeared. In all these cases, 

 it is plain that there were present in parts affected, the 

 determinants, to use Weismann's term, not only of the 

 normal parts, but also of parts, which, under normal 

 conditions, would never have appeared there ; and 

 these new parts growing in the unwonted places bore 

 the normal species-stamp as characteristically as sim- 

 ilar parts grown in their normal places. It can hardly 

 be supposed that the architecture of the germ-plasm 

 contains special determinants to be ready for occur- 

 rences so casual, especially as these are called into 

 existence by circumstances quite foreign to the normal 

 environment of the organisms. On the other hand, 

 the facts are consonant with Hertwig's belief that, as 

 all division is heirs-equal division, all the species- 

 characters that depend upon cells are latent in every 

 cell. 



