336 Dr. D. H. Scott [May 15, 



relative superiority of one group over another in the struggle for 

 life. Everything depends on the conditious of the contest. 



In the simpler of the higher Cryptogams, such as ordinary Ferns, 

 the spores are all of one kind and on germination give rise to an 

 independent j^lantlet, the prothallus, on which the sexual organs are 

 borne. Fertilisation requires the presence of water for the actively 

 moving male cells, the spermatozoids, to swim in. This condition 

 may be something of a handicap to the plant, but if water is present, 

 reproduction is fairly well ensured. In the more advanced spore- 

 plants, such as the Selaginellas so commonly grown in our green- 

 houses, the differentiation of the sexes begins earlier, for the spores 

 themselves are of two kinds. There are numerous male spores of 

 very small size (microspores) and comparatively few female spores of 

 relatively large size (megaspores). In the group of the Water-ferns 

 (Hydropteridese) only one of these large spores is produced in each 

 spore- sac, which then, if provided with a special envelope, as in 

 Azolla, may closely simulate a seed. 



In the microspores, the prothallus is scarcely developed ; the spore 

 has practically nothing else to do but to produce the spermatozoids. 

 On the female side, provision has to be made for the nutrition of the 

 embryo, and here there is a comparatively bulky prothallus, though 

 as compared with that of the Ferns, it tends to lose the character of 

 an independent plant and to become a mere storehouse of food- 

 materials. There are certain obvious advantages in this heterosporous 

 condition. The male spores are kept small for easy dispersal and 

 can be produced in correspondingly large numbers. The prothallial 

 tissue is economised and only formed where it is wanted, i.e. in 

 connection with the egg-cells from which the embryos arise. 



The differentiation of microspores and megaspores is in fact com- 

 parable to that earlier differentiation of minute mobile spermatozoids 

 and large stationary ovum, which took place far back in the history 

 of both animals and plants, and laid the foundation of sex. 



At the same time, the heterosporous arrangement, as we find it in 

 Cryptogams, puts a new obstacle in the way of the successful 

 accomplishment of the act of fertilisation. In order that this may 

 happen, it is necessary that the two kinds of spores should germinate 

 together as well as in the presence of an adequate water supply. The 

 necessary association of the large and small spores is, as a rule, left 

 to chance, the small spores being produced in enormous numbers, so 

 that the chance may be a good one. 



In the case of the great cryptogamic trees of the Palesozoic period, 

 the difficulty must have been a serious one. We know that their large 

 and small spores often differed in mass in the proportion of at least 

 100,000 to 1, and when bodies of such diverse weights were scattered 

 by the wind from the tops of lofty trees, the chances must have been 

 enormously against their coming to rest at the game spot. It was 

 perhaps to this difficulty that the series of adaptations leading up to 

 seed-formation owed their first inception. 



