228 THE INDUCTIONS OF BIOLOGY. 



both that this introversion of the cambium-layer may be 

 considered as due to failing nutrition, and that the ovulea 

 growing from its introverted surface (which would have been 

 its outer surface but for the defective nutrition) are extremely 

 aborted homologues of external appendages — either leaves 

 or lateral axes : the essential organs of fructification thus 

 arising where the defective nutrition has reached its extreme.* 

 To all which let us not forget to add, that the sperm -cells and 

 germ-cells are formed at the very ends of the organs of fruc- 

 tification. 



Those kinds of animals which multiply by heterogenesis, 

 present us with a parallel relation between the recurrence of 

 gamogenesis and the recurrence of conditions unfavourable to 

 growth — at least, this is shown where experiments have 

 thrown light on the connexion of cause and eflTect ; 

 namely, among the Aphides. These creatures, hatched from 

 eggs in the spring, multiply by agamogenesis throughout 

 the summer. When the weather becomes cold, and plants 

 no longer aflEbrd abundant sap, perfect males and females 

 are produced ; and from gamogenesis there result fertilized 

 ova. But now observe that beyond this evidence, wo 

 have much more conclusive evidence. For it has been shown, 

 both that the rapidity of the agamogenesis is proportionate 

 to the warmth and nutrition, and that if the temperature and 



* It appears that botanists do not agree respecting the homologies of the 

 ovules: some thinking that they are rudimentary foliar organs, and others that 

 they are rudimentary axial organs. Possibly the dispute wiU prove a bootless 

 one ; since there seems evidence that ovules may be transformed into either one 

 or the other. Mr Salter's paper, lately referred to, shows that they may 

 graduate into stamens, which are foliar organs ; and the case of the Foxglove, 

 which I have described above, shows that they may develop into flower-ouds, 

 nhich are axial organs. I would venture to suggest, that the conflicting evidence 

 can be reconciled, only by regarding ovules as the homologues of lateral append- 

 ages ; and considering a lateral appendage 3s composed of a leaf, plus a rudiment- 

 ary axis, either of which may abort. This is the view which seems countenanced 

 by development ; since, in its first stage, a lateral bud, whence a lateral append- 

 age arises, shows no division into rudimentary leaf and rudimentary axis ; and 

 it is to the lateral bud in this first stage, that the seed-bud or ovule is homo 

 logous. 



