136 THE EVOLUTION OF PLANTS 



partly on the fact that some of their races were 

 incomparably better developed in early geological 

 times than at present, so that their past history 

 is well worth tracing. 



We may shortly define the higher Spore-plants, 

 with which alone we are concerned, as those in 

 which there is a regular alternation of sexual 

 and asexual individuals, the individual bearing 

 the sexual organs being always relatively small 

 and simple (the prothallus), while that bear- 

 ing the spores is highly developed and consti- 

 tutes the plant itself, as ordinarily known to 

 us. Thus in a Fern, a Lycopodium or a Horsetail 

 the plant is strictly asexual, bearing the spores, 

 which in these cases are all of one kind. The 

 prothallus or sexual generation, bearing the 

 antheridia and archegonia (see above, p. 121), 

 is always small and may never have been seen 

 even by most people to whom the plant itself 

 is familiar. In fact, in the case of Lycopodium 

 and the Adder's Tongues, it is only within the 

 last quarter of a century or less that the sexual 

 individual has become known to science. 



In certain of the higher Cryptogams already 

 referred to (p. 122), namely, in the peculiar group 

 of the Water-ferns, and in Selaginetta and Isoetes 

 among the allies of Lycopodium, a certain sexual 

 distinction is already manifest on the plant 

 itself, for here the spores are of two kinds. In 



