GREAT STEPS IN ORGANIC EVOLUTION 875 



and how did plant-life attain to such exuberance before there were 

 any seeds ? The answer to the second half of the question is easy — 

 we have only to shake one of these withering fern-fronds on a sheet 

 of black paper. Then we see a shower of "spores", which used to be 

 called "fern-seed". But they are altogether different from seeds, 

 for they are single cells, as light as air, whereas seeds are embryo 

 plants, usually well equipped with food and well protected by firm 

 envelopes. But while we emphasise this fact, that the older flowerless 

 plants, like mosses and ferns, multiplied themselves by unicellular 

 spores, whereas the subsequent flowering plants multiplied by seeds, 

 which already contain complex embryos when they are set adrift, 

 there is in this a suppressio veri which obfuscates the whole problem. 



For when the spore of a fern sinks to the moist soil, it develops, 

 not into another fern plant like the parent, but into a small green 

 disc or prothallus, which bears sex-organs. From the fertilised 

 egg-cell of this unfamiliar sexual phase there develops the spore- 

 bearing fern-plant we all know so well. This is alternation of 

 generations, which is characteristic of the plant kingdom, and also 

 occurs in a somewhat different way in some animal types, e.g. among 

 the zoophytes, the Medusae, the flukes, and the Tunicates. Its wide- 

 spread occurrence among plants was discovered by the genius of 

 Hofmeister, a botanical music-dealer, who left a deep mark on 

 biological science. 



The alternation is between an asexual spore-producing sporophyte, 

 say the ordinary fern-plant, and a sexual gametophyte, like the fern- 

 prothallus, producing eggs and sperms. From the fertilised egg-cell 

 the sporophyte develops, and so the cycle continues. The sequence 

 may be defined as the alternate occurrence in one life-history of two 

 different forms differently produced. Of recent years it has been 

 shown that the cells of the sporophyte, say the ordinary fern-plant, 

 have twice as many nuclear chromosomes as the cells of the gam- 

 etophyte, say the fern-prothallus. So it is a deep difference, though 

 among some Algae the two phases are superficiallj^ very like one 

 another. 



Alternation of generations began in the sea, and for millions of 

 years all went weU. But when some aquatic plants established them- 

 selves on land the gametophyte began to be badly handicapped. 

 Thus one drawback was that the male-cell was suited for swimming 

 freely in water and could not reach the egg-cell in any other way. 

 Moreover, it is plain that the dry land does not afford such a safe 

 and soft cradle for egg-cells and embryos as the water does. And 

 thus, as Prof. F. O. Bower showed in a masterly way many years 

 ago, there began, in increased adaptation to terrestrial life, a series 

 of changes that led to the seed-plants. This came about through a 

 gradual dwindling of the gametophyte and an increasing ascendancy 

 of the sporophyte. To what Hofmeister had described. Bower gave an 



