CAUSES OF THE PHENOMENA OF LIFE. 37 



individual Comatula is the Hydrozoic stock plus all the Me- 

 dusae which proceed from it. 



No doubt it sounds paradoxical to speak of a million of 

 Aphides, for example, as parts of one morphological individ- 

 ual ; but beyond the momentary shock of the paradox no 

 harm is done. On the other hand, if the asexual Aphides 

 are held to be individuals, it follows, as a logical consequence, 

 not only that all the polyps on a Cordylophora tree are 

 " feeding individuals," and all the genital sacs " generative 

 individuals," while the stem must be a " stump individual," 

 but that the eyes and legs of a lobster are " ocular " and 

 " locomotive individuals." And this conception is not only 

 somewhat more paradoxical than the other, but suggests a 

 conception of the origin of the complexity of animal struct- 

 ure which is wholly inconsistent with fact. 



IV. ^Etiology. 



Morphology, distribution, and physiology, investigate and 

 determine the facts of biology. ^Etiology has for its object 

 the ascertainment of the causes of these facts, and the ex- 

 planation of biological phenomena, by showing that they con- 

 stitute particular cases of general physical laws. It is hardly 

 needful to say that aetiology, as thus conceived, is in its in- 

 fancy, and that the seething controversies, to which the 

 attempt to found this branch of science made in the " Origin 

 of Species " has given rise, cannot be dealt with in this place. 

 At most, the general nature of the problems to be solved, and 

 the course of inquiry needful for their solution, may be indi- 

 cated. 



In any investigation into the causes of the phenomena of 

 life, the first question which arises is, Whether we have any 

 knowledge, and if so, what knowledge, of the origin of living 

 matter ? 



In the case of all conspicuous and easily-studied organ- 

 isms, it has been obvious, since the study of Nature began, 

 that living beings arise by generation from living beings of 

 a like kind ; but, before the latter part of the seventeenth cen- 

 tury, learned and unlearned alike shared the conviction that 

 this rule was not of universal application, and that multitudes 

 of the smaller and more obscure organisms were produced by 

 the fermentation of not -living, and especially of putrefying 

 dead matter, by what was then termed generatio cequivoca 

 or spontanea, and is now called abiogenesis. Redi showed 



