D,— ZOOLOGY. 89 



plant life has succeeded in making some progress is the nari'ow belt 

 along the shores, where a fixed life is possible, but this belt, limited 

 by the amount of liglit which penetrates, extends only to a depth of 

 about 15 fathoms. The available area is further restricted to rocky 

 and hard bottoms, and is therefore nowhere great. This is the wave- 

 lashed region of the brown and red sea-weeds. In the brown sea- 

 weeds a history can still be traced,'^ from the fixture of an autotrophic 

 flagellate to the building up, by laying cell on cell, of the essential 

 structures which afterwards, on transmigration to the land, reached 

 their climax in the forest tree. 



But if the flagellate thus rose and gave origin to the flora of the 

 land, it also degenerated, for it adopted a parasitic habit, livmg in 

 and directly absorbing already formed organic matter. In this way 

 the bacteria arose, whose activities in so many directions influence the 

 life of to-day. This view exceeds in probabilit}', I think, the suggestion 

 often put forward, ^"^ that it is to the simpler bacteria we must look for 

 the first beginnings of life. 



After this digression on the botanical side we must return to the 

 primitive ccelenterate and see on what lines evolution proceeded in 

 t'he animal world. As a purely plankton organism, swinmiing freely 

 in the water, the progress of the ccelenterate was not great, and 

 reached, as far as we know, no further than the modern Ctenophore. 

 The Ctenophore seems to represent the culminating point of the 

 primary progression of pelagic animals, which derived directly from 

 the autotrophic flagellate. Further evolution was associated with an 

 abandonment by a ccelenterate-like animal of the pelagic habit, and 

 the establishment of a connection with the sea bottom, either by fixing 

 to it, by burrowing in it, or by creeping or running over it. At a 

 later stage many of the animals which had become adapted to these 

 modes of life developed new powers of swimming, and thus gave rise 

 to the varied pelagic life which we find in the sea to-day; but this 

 must be regarded as secondary, the primary pelagic life, so far as 

 adult animals were concerned, having ended with the evolution of 

 tlie Ctenophore. ^^ Such is the teaching of embryology, the history of 

 the race being conjectured from the development of the individual. 

 In group after group of the animal kingdom, when the details of its 

 embryology become known, the indications are the same — first the 

 active spermatozoon, reminiscent of the plankton flagellate, then the 

 pelagic larval stage, recalling the ccelenterate, and then a bottom- 

 living phase. 



" Church, Botanical Memoirs. No. 3. Oxford, 1919. 



" Osboni, ' The Origin and Evolution of Life,' 1918. Waksman and Joffe. 

 ' Micro-organisms concerned in tlie Oxidation of Sulphur in the Soil,' Journal 

 of BacUrioloiiy, VII. 2, March 1922. The authors claim that Thiobacillus 

 Ihiooxidans will grow in solutions containing no organic matter. In view of 

 the minute traces of organic matter that suffice for the growth of bacteria and 

 moulds, care must be taken, however, in drawing conclusions from experiments 

 made in flasks or tubes closed in the ordinary' way with cotton-wool plugs and 

 subsequently sterilised in flowing steam. 



" There is perhaps a possibility that further knowledge of the embryology 

 of Sagitta and its allies might make it necessary to modify this suggestion. 



