100 



SCIENCE. 



[N. S. Vol. XVII. No. 420. 



An alternation of generations is hinted 

 at among some of the green algffi, but never 

 becomes sliarply defined as it is in the 

 archegoniates. Among the red algte, liow- 

 ever, it becomes clearly marked, and also 

 in many fungi. In both of the latter eases 

 it is extremely probable that we have to do 

 merely with analogies, as there is not the 

 slightest evidence of any genetic connec- 

 tion between either of these groups and the 

 archegoniates. 



With the green algffi, however, the case 

 is somewhat different, and it is highly prob- 

 able that the earliest archegoniates arose 

 from some forms not very different from 

 Coleochcete, a green alga in which the fer- 

 tilized egg gives rise to a very simple 

 sporophytic structure. 



The increase in the output of the zygote, 

 or fertilized egg, due to its division into 

 a number of spores, instead of forming at 

 once a single new individual, is an evident 

 advantage which becomes increasingly im- 

 portant as the gametophyte assumes the 

 character of a terrestrial plant, and the 

 chances of fertilization, which requires the 

 presence of water, become correspondingly 

 lessened. 



There are two theories as to the origin of 

 the alternation of generations among the 

 archegoniates, the 'homologous' and 'anti- 

 thetic. ' The first holds that the non-sexual 

 sporophyte is a direct modification of the 

 gametophyte and probably arose from it 

 as a vegetative outgrowth. The antithetic 

 theory holds that the sporophyte always, 

 in normal cases, arises from the fertilized 

 ovum, and is a further development of the 

 zygote Avhich has arisen in response to the 

 requirements of a terrestrial existence. 

 There is not time here to consider at length 

 the relative merits of these two theories. 

 In a special paper before the section, I 

 hope to bring this matter up for discus- 

 sion. For present purposes I shall assume 



that the latter (antithetic) view is the 

 correct one. 



As the ancestors of the archegoniates left 

 their original aquatic habitat, the question 

 of the water supply became of the first im- 

 portance. All of these lower land plants 

 have retained many of their original char- 

 acteristics, among them the development 

 of motile male cells (spermatozoids), 

 which require free water in order that 

 they may reach the egg-cell and fertilize it. 

 That is, the plants are, to a certain extent, 

 amphibious, and must return to the water 

 in order that fertilization may be effected. 

 It is very clear, then, that anything which 

 tends to increase the number of spores re- 

 sulting from the developed zygote will be 

 advantageous, rendering a single fertiliza- 

 tion more and more effective. 



The alternation of sexual and non-sexual 

 plants among the green algse is not sharply 

 marked, and has been shown to be largely 

 a matter of nutrition. Nevertheless, as 

 already mentioned, there is a hint of an 

 alternation of generations in certain forms 

 like the higher ConfervoidcEe. In these 

 the germinating zygote produces a larger 

 or smaller number of zoospores, which give 

 rise to as many new individuals. From 

 some such form as these in all probability 

 the primitive archegoniates arose. As these 

 became distinctly land plants, the motile 

 zoospores resulting from the zygote of the 

 algffi gave place to the non-motile spores 

 characteristic of the terrestrial archegon- 

 iates ; but of any transitional forms we are 

 quite ignorant, and the gap between algffi 

 and archegoniates is a very deep one. 



The gradual specialization exhibited by 

 the existing liverworts and mosses is famil- 

 iar to all botanists, and will only be briefly 

 discussed here. Enough to say that from 

 the simplest type, a globular mass of spores, 

 with almost no sterile tissue developed, 

 such as occurs in the Ricciaceffi, there are 



