564 THE BEGINNINGS OF LIFE. 



the parent form. And where development takes place 

 by such successive stages — which, as it were, reproduce 

 or copy previous current heterogenetic changes — the 

 same tendency to multiply by an asexual process is 

 manifested by the several forms which represent the 

 different developmental stages, as was previously dis- 

 played by the several heterogenetic products. 



Heterogenesis, therefore, would thus appear to be 

 the essential underlying cause of all developmental 

 transformations or metamorphoses- and, moreover, 

 the hitherto unexplained phenomena of ^alternate 

 generation' may, perhaps, be deemed to receive a 

 rational and approximate interpretation 1. 



^ There would thus be three distinct modes by which sexual repro- 

 duction may make its appearance amongst the ' Ephemeromorphs ' : — 



(a) It may occur as it were ' accidentally,' at long and altogether 

 irregular intervals, during some of the later stages in a series of ephe- 

 meromorphic developments — and then in a very rudimentary manner. 

 The best instances of this are, perhaps, to be found amongst Fungi. 

 Thus I would suggest that a relationship of this kind probably obtains 

 between Uredo, ^cidium, and Uromyces as a possible final form in 

 which rudimentary sexual organs may become differentiated. 



ih) It may occur, similarly, in one of the later stages of a series of 

 ephemeromorphic transformations, though such changes may subse- 

 quently tend to recur so as to produce a case of cyclical or ' indirect ' 

 homogenesis. Portions of ephemeromorphic life are, as it were, thus 

 nipped off and preserved, so as to constitute the different kinds of 

 ' alternate generation ' which are known to prevail both amongst animal 

 and vegetal forms of life. 



(c) It also occurs when an ephemeromorphic series ends abruptly — 

 i. e. when by some final process of heterogenesis an organism is at 

 once produced which subsequently develops sexual characters, and 

 thenceforth multiplies by the homogenetic method. In this case the 



