460 Proceedings of the Royal Physiccd Society. 



and birds ; but their surroundings have a much more restricted 

 character. Thus we find sponges, corals, and shells moulded 

 as regards their external form by currents, just as trees are 

 blown into shape by the wind. But the variations so pro- 

 duced are, comparatively speaking, small details. Dall (206) 

 notices in regard to deep-sea molluscs, where the changes in 

 environment are obviously few, that the number of individual 

 variations in form and sculpture is often great, but the 

 number of " flexible species " few. Similarly the compara- 

 tively long-lived and passive leaves of plants are subject to 

 numerous variations, in many cases doubtless environmental, 

 while the short-lived and relatively active reproductive 

 leaves are much less subject to detailed environmental 

 variations. Differences in the reproductive organs of plants 

 can indeed be rarely associated with environmental influence 

 except in the cases of mechanical modifications due to insects. 

 As with less passive organisms, so with less vegetative parts, 

 they are less in the grasp of their environment. 



Yet active organisms come within a richer and more 

 variable circle of influences, some of which have doubtless 

 had a directly modifying action ; and it is significant in this 

 connection to notice that the passive forms, with limited 

 environment, are relatively unprogressive. In appreciating 

 the limitations of forms like sponges, echinoderms, or bivalves, 

 while the greater part of the drag may be due to their in- 

 herited constitution, the restriction of environmental influence 

 must not be overlooked as an important part of the nemesis 

 of passivity. 



At different ages too, as above suggested, the environmental 

 influences will vary. It is with young forms that investi- 

 gators have most successfully experimented. The number of 

 adaptive larval characters is familiarly great. On the other 

 hand there are many features in old animals which suggest 

 that after the period of active maturity, of thrust and parry, 

 is past, the organism conies again markedly under the power 

 of its environment. Thus it was that Treviranus (258) dis- 

 tinguished two more passive periods of vita minima, one on 

 each side of the active vita maxima of reproductive and vital 

 maturity. 



