250 PHYSIOLOGICAL DEVELOPMENT 



that conduce to growth are nearlj' equilibrated by the forces 

 that resist growth (§ 78) ; and the induction that in plants, fer- 

 tilized germs are produced at places where there is an approach 

 towards this balance, we found to be in harmony with the 

 deduction that an advantage to the species miist be gained 

 by sending off migrating progeny from points where nutri- 

 tion is failing. Other things equal, failure of nutrition 

 may be expected in parts that have the most remote or most 

 indirect access to the materials furnished by the roots — 

 materials that have to be carried great distances by a very 

 imperfect apparatus. The ends of lateral axes are therefore 

 the probable points of fructification, in .aggregates of the 

 third order that have taken to growing vertically. But 

 if these points, at whicli nutrition is failing, are also the 

 points at which the colours inherited from lower types are 

 likely to recur in more marked degrees than elsewhere ; then 

 we may infer that the organs of fructification will not un- 

 frequently co-exist with such colours at the ends of such 

 axes. How may the resulting contrast between the older 

 fronds and the fronds next the germ-producing organs be 

 increased ? If uninterfered with it would be likely to di- 

 minish. These traits inherited from remote ancestry, might 

 be expected slowly to fade away. How, then, is the intensi- 

 fication of them to be explained ? 



If a contrast of the kind described favours the propagation of 

 a race in which it exists, it will be maintained and increased ; 

 and if we take into account an agency of which Mr. Darwin has 

 shown the great importance — the agency of insects — we shall 

 have little diflSculty in understanding how such a contrast 

 may facilitate propagation. We cannot, of course, here -assume 

 the agency of insects so specialized in their habits as Bees and 

 Butterflies; for their specialized habits imply the pre-exist- 

 ence of the contrast to be explained. But there is an insect- 

 agency of a more general kind which may be fairly counted 

 upon as coming into action. Various small Flies and 

 Beetles wander over the surfaces of plants in search of 



