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 Palaeophytology, 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-eycles. This must take due cognisance of 
the other branches of study, but will still rest upon its own footing 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, 
