COMPARISON OF LIFE-CYCLES 



as indeed there is good reason to ^ee 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 in 

 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 



