342 PHYSIOLOGICAL DEVELOPMENT. 



liquid, having been once established, survival of the fittest 

 would suffice for its gradual extension and its successive 

 modifications. But what were the early stages of the con- 

 tractile tube, while it was yet not sufficiently formed to help 

 circulation, and while it must nevertheless have had some 

 advantage without which no selective process could go on? 

 The question seems insoluble. To another part of 



the question, however, an answer may be ventured. If we 

 ask the origin of these ramifying channels which, first 

 appearing as simple lacunae, eventually become vessels 

 having definite walls, a reply admitting of considerable 

 justification, is, that the currents of nutritive liquid forced 

 and drawn hither and thither through the tissues, themselves 

 initiate these channels. We know that streams running 

 over and through solid and quasi-solid inorganic matter, 

 tend to excavate definite courses. We saw reason for con- 

 cluding that the development of sap-channels in plants 

 conforms to this general principle. May we not then 

 suspect that the nutritive liquid contained in the tissue 

 of a simple animal, made to ooze now in this direction and 

 now in that by the changes of pressure which the animal's 

 movements cause, comes to have certain lines along which it 

 is thrust backwards and forwards more than along other 

 lines; and must by repeated passings make these more and 

 more permeable until they become lacuna? Such actions 

 will inevitably go on; and such actions appear competent to 

 produce some, at least, of the observed effects. The leading 

 facts which indicate that this is a part cause of vascular 

 development are these. 



Growths normally recurring in certain places at certain 

 intervals, are accompanied by local formations of blood-ves- 

 sels. The periodic maturation of ova among the Mammalia 

 supplies an instance. Through the stroma of an ovarium are 

 distributed innumerable minute vesicles, which, in their early 

 stages, are microscopic. Of these, severally contained in their 

 minute ovi-sacs, any one may develop: the determining 



