COMPARISON OF LIFE-CYCLES 9 
as indeed there is good reason to see that it was; but it consisted still in 
the fusion of two cells together, bringing, as we believe generally, and see 
proved already in so many cases, a doubling of the chromosome-number 
aS a consequence. Seeing sexuality of this nature a constantly recurring 
feature in the life-cycle of various definite phyla leads to the conclusion 
‘that in those phyla it was also constant during their descent. Similarly, 
a reduction of chromosome-number has been found to be regularly associated 
with normal spore-production, and spore-production is found to be a 
constantly recurring event in large series of plants. In these it is concluded 
that reduction and spore-production have also been constantly recurring 
incidents throughout the descent of those series. It is hardly right to 
designate this opinion as an assumption: it seems rather to be a natural 
and valid outcome of comparative study. But if, on the other hand, such 
constancy of the leading events of the life-cycle in any phylum during 
descent were to be clearly disproved, then it will follow with equal clearness 
that the comparative argument based upon such facts will have to be 
revised for that phylum. It may seem hardly necessary to put down zm 
extenso reasoning which is so obvious; but, on the other hand, it is well 
to see clearly the basis upon which the main argument will proceed. The 
constancy of the events of sexuality and of spore-production in normal 
life-cycles of the several ascending series of green plants is itself the cardinal 
point of the theory to be advanced in relation to the origin of a Land-Flora. 
In so far as inconstancy of either of these events occurs in them it will 
be shown that there is good reason to believe such exceptions to be of 
relatively late origin. : 
The further facts which form the basis of Comparative Morphology 
include those relating to the mature external form of the plant, as seen 
in the successive phases of the individual life-cycle: the internal structure, 
as shown by its anatomical study: the form and structure of the parts 
involved in propagation, and the embryology of the individual. Such 
facts relating to living organisms are to be read in the light of comparison 
with the fossils, and the validity of any conclusions tested as far as 
possible according to the results of physiological experiment. 
It has been customary from the earliest times of natural classification 
to group together as akin, according to their degree of similarity, those 
organisms which correspond in form. Such alliances, long ago recognised, 
received a new significance in the light of evolutionary theory: the likeness 
thus comes to be attributed to community, of descent, the nearness of the 
kinship being held proportional to the similarity of form, structure, and 
development of the individual. It is essential, however, to bear in mind 
always: that this is only an hypothesis, incapable of complete proof under 
present conditions of study, and that the extent of direct evidence as 
yet available is small indeed. It is true that variation in different degrees 
is widespread: that, whatever the causes or methods involved, new races 
may :be, and indeed have been established, which come true in more or 
