SECRETIVE TISSUES. 147 



Witli reference to the continnous flow of nectar, I would 

 draw some analogy from animal secretions. Mr. Darwin, in 

 speaking of the cow, observes * : " We may attribute the 

 excellence of our cows and of certain goats, partly to the 

 continued selection of the best milking animals, and partly 

 to the inherited effect of the increased action, through mans art, 

 of the secreting glands.^' This fact, recorded in the last 

 sentence, which I have italicized, is only one example of the 

 general principle of increase of growth by use, which I take 

 to be strictly analogous to what takes place in the vegetable 

 kingdom. And we may notice, in its special application to 

 the formation of glands and other structures by mechanical 

 irritation, that it is none other than a mechanical irritation 

 which keeps up the secretion of milk for prolonged periods. 



The common or physical basis of vegetable life, namely 

 protoplasm, is very nearly f indistinguishable in its properties 

 from that of animals. Their behaviour is every day being 

 proved to be not only similar but identical in the two 

 kingdoms. The effects, under mechanical irritations and 

 strains, of nutritive matters of the same kind, of poisonous 

 substances, of electricity, etc, all show that the bond which 

 unites the animal and vegetable kingdoms together is of one 

 and the same nature, and that the links of the chain are 

 forged out of this common basis of life. 



It is not to be wondered at, then, but rather to be antici- 



same direction. In the case of the eye, I take that cause to be light. 

 In the case of an irregular corolla or the pitcher of Nepenthes, I assume 

 it to be insects {Tr. Lin. Soc, xxii., p. 415 j An7i. Sci. Nat., 4 ser., xii., 

 p. 222). 



Conversely, in the absence of light the eye vanishes ; in the absence 

 of insects, corolla, honey, etc., go ; so that negative evidence tends to 

 support the positive in all cases alike ; see Or. of Sp., 6th ed., p. 110. 



* Anim. and PI, tender Dom., ii., p. 300. 



t See Journ. Roy. Micr. Soc. 1887, 771. 



