THE EVOLUTION OF PLANTS. 609 



of the denizens of the ocean have no need of segmentation into stem and leaf. 

 Indeed, such a metamorphosis for this purpose would be a disadvantage, anythino- 

 but a progressive development. Similarly is it inconceivable, from all we know 

 of the relations between external conditions and the form of an organism, that 

 a Fern (for instance), unable to accomplish its fertilization in dew or rain-water, 

 should in consequence depart from its usual habit and strike out a new line of 

 metamorphosis. Thus we may conclude^ that the development of the individual 

 (Ontogeny) cannot be regarded as an epitome of the ancestral history or line of 

 descent of that individual, and, further, that ontogeny gives no support to the 

 assumption of a ramifying phylogenetic tree starting with simple forms and ending 

 with complex much-differentiated ones. 



The results of developmental investigations showing a marked similarity in the 

 form of organs serving similar purposes in the most different groups of plants have 

 been brought forward in support of the assumption that Flowering Plants have 

 arisen from simple Cellular Plants by a series of progressive transformations. 

 Though these organs are in some groups of more simple, in others of more complex 

 structure, their similarity is unmistakable; it is upon this that the view is widely 

 based that organisms exhibiting similar organs have been derived from one another. 

 But this inference is inadmissible. The similarity in question finds a simpler ex- 

 planation as the expression of the attainment of a common object. Thus fertiliza- 

 tion consists in the coming together and uniting of two portions of protoplasm 

 which have originated at a distance from one another; the similarity of ways and 

 means in attaining this object are obvious. In one case water is employed as a 

 means, in another the air. There is, truly, a difference in detail, but the general 

 similarity remains. From this general similarity all we are justified in inferring is 

 that the organisms in question all reproduce themselves by fertilization, not that 

 they have a common origin. 



This conclusion leads to the question whether, in view of the diversity of the 

 organs of fertilization, several distinct stems of plants may not have co-existed 

 all along. We know from observation and experiment that new forms do not 

 as a rule arise from offshoots, but from fruits. New groups of plants might thus 

 (so it was said) arise from existing ones solely by the sexual method. Complex 

 Thallophytes might arise from the fruits of simpler ones. Ferns from the fruits 

 of Mosses, and so on. Assumptions of this kind belong to a period at which the 

 phenomena of fertilization and fruit-production, especially in the Cellular Plants, 

 were only very imperfectly understood. No Botanist nowadays would suggest the 

 possibility of a Spirogyra or a Ulothrix, or an (Edogoniu7)i, or a Stonewort, arising 

 from the fruit of a Vaucheria. It might well happen that a new species could 

 arise by the crossing of two dichogamous species of Vaucheria, but this new form 

 would be essentially a Vaucheria, and its methods of fertilization would be essen- 

 tially similar to that of its parent-forms. It is likewise as impossible for the male 

 protoplasm of a Vaucheria to enter the chamber in which the conjugation of the 



' See editorial note prefacing this volume. 



