PALEOBOTANY BEERY. 377 



Another early evolutionary advance was the origin of heterospory 

 in the originally homosporous lines of descent. Reproduction by 

 spores was, and still is, essentially an aquatic process, and hence de- 

 pendent almost entirely upon external and uncontrollable conditions. 

 By developing two kinds of spores the megaspores could be provided 

 with a better chance of survival, while the microspores could be effi- 

 ciently produced in much greater numbers, and thus render the chances 

 of fertilization more favorable. The next step, approached through 

 a series of forms that retained the megaspore for a gradually length- 

 ening period, during which the parent plant furnished the needed 

 nutriment and water and evolved an apparatus for facilitating the 

 access of the sperm to the egg cell, leads naturally to the organization 

 of true seeds, which was early approximated in such diverse stocks as 

 the Lepidophytes and Pteridophytes, and perfected in the Pterido- 

 spermophytes. It is perhaps unnecessary to dwell on the advantages 

 of the seed habit, the supremacy of its possessors in the later period of 

 geological history, and at the present time, is conclusive evidence of 

 the advantage it gives in the struggle for existence. It may be com- 

 pared to the lengthening of the period of infancy, which is a factor of 

 such importance in the most advanced human races, and seed and 

 spore methods of reproduction may be compared with the viviparity 

 of the higher mammals as contrasted with the oviparity of such ani- 

 mals as most of the fishes or the pelecypod mollusca. 



The question of the origin of the characteristic alternation of 

 generations is not directly a paleobotanical question. The absence 

 of Bryophytes from the early floras and the general character of the 

 oldest known land floras are entirely opposed to the derivation of the 

 sporophyte from a sporogonium. The view that it was derived from 

 a branched thallus has several supporters (Tansley, Lang). On the 

 other hand, as Coulter forcibly puts it, there is no reason why an 

 erect leafy axis bearing neither spores nor gametes is not quite as 

 supposable as the appearance of a sporophore with neither gametes 

 nor leaves, or a gametophore with neither spores nor leaves. On 

 such a view, leaves would be primary structures — a view with much 

 historical probability. Consequently the evolution of cones was se- 

 quential — a view in accordance with the older morphology. 



PRE-DEVONIAN FLORAS. 



Except for some very inconclusive objects, many doubtful, and a 

 considerable number of well-authenticated algae (such as Core- 

 matocladus, Chaetocladus, Callithamnopsis, Primicorallina from the 

 Trenton limestone of New York and Wisconsin; Nematophycus and 

 Pachytheca from America and Europe), no fossil plants are known 

 from the pre-Devonian. The supposed Silurian flora described by 

 Dawson and Matthew from New Brunswick, and that described by 



