8 SCOPE OF COMPARATIVE MORPHOLOGY 



when he said that " Morphology includes such phenomena as are not 

 yet physiologically understood." He further indicates that the separation 

 of the two points of view has not any foundation in the nature of the 

 case, but it is only a preliminary aid to a clear view amid the multiplicity 

 of phenomena. The limits between morphology and physiology must 

 necessarily fall away as advances are made. But meanwhile Morphology 

 must continue to exist, even though it is not and cannot be an exact 

 science: it deals comparatively with phenomena imperfectly explained as 

 regards their origin in the individual or the race. The history of develop- 

 ment of plant-form is an ideal to be approached experimentally, and the 

 final object will be not merely a knowledge of the phylogenetic development, 

 but of the very essence and cause of the development itself. It will be 

 obvious how far present phylogenetic theory falls short of this ideal of 

 Causal Morphology, but that is no sufficient reason for discontinuing its 

 pursuit as a progressive study. 



For the present the comparative study of plant-form from the point 

 of view of descent, as exhibited in the various phases of the individual 

 life-cycle, must be pursued as in itself a substantive branch of the science : 

 it is clear from what has been said above that it is not co-extensive with 

 either Palaepphytology, Plant-Geography, or Plant-Physiology : nevertheless 

 it overlaps with all of these, and must be liable to be checked by the 

 results of any of these branches. Furthermore, the extension of knowledge 

 of any of these branches will inevitably lead to further overlapping, till 

 in the end the knowledge derived from the various methods of investigation 

 should coincide in conclusions which will be general for them all, and 

 constitute a true perception of the evolutionary story. But at the moment 

 this consummation is so far from being attained that there is still room 

 for the theoretical treatment of the evolution of plants as based on the 

 formal comparison of their life-cycles. This must take due cognisance of 

 the other branches of study, but will still rest upon its own foojting of 

 fact and conclusion. 



There is one assumption involved in such comparative study which 

 should be clearly apprehended and considered, rather than tacitly passed 

 over. An evolutionary argument based on comparison of life-cycles is 

 only valid if the organisms compared have retained the main incidents 

 in their individual life unchanged throughout descent. In the main argu- 

 ment of this work, the assumption is deliberately made that such constancy 

 existed, or, rather, the argument proceeds upon the conclusion derived 

 from broad comparison, that the main incidents once initiated have been 

 pertinaciously retained. It may be held, and reasonably defended, that 

 sexuality may have arisen in many distinct phyletic lines. It is not our 

 present purpose to distinguish those different origins, or defend their 

 distinctness. But comparison leads us to conclude that, once initiated in 

 an evolutionary sequence, sexuality remained throughout descent substantially 

 the same process in normal life-cycles. It may be modified in mechanism, 



