45o 



NA TURE 



[September 30, 1922 



from elements occurring in sea-water of a chlorophyll- 

 bearing flagellate, capable of manufacturing its own 

 nourishment and able to multiply indefinitely by the 

 simple process of dividing in two. If we assume only 

 one division during each night as a result of the day's 

 work in accumulating food material, such an organism 

 would be able in a comparatively short space of time 

 to occupy all the natural waters of the world. But 

 here we are met by a difficulty which is not easily 

 overcome. Chlorophyll, the photocatalyst, the most 

 essential factor in the building up of the new organic 

 matter, is itself a highly complex organic substance, 

 and in any satisfactory theory its original formation 

 and its constant increase in quantity must be accounted 

 for. Lankester 7 has maintained that chlorophyll must 

 have originated at a somewhat late stage in the develop- 

 ment of organic life, and has suggested that earlier 

 organisms may have nourished themselves like animals 

 on organic matter already existing in a non-living 

 state. An alternative hypothesis, which in view of 

 the recent work seems more attractive, is to suppose 

 that the earlier organisms were either activated by 

 some simpler photocatalyst, or that they received the 

 necessary energy at suitable frequency directly from 

 some outside source. 



It must not be forgotten, also, that at the time 

 these developments were taking place the conditions 

 of the environment would in many ways have been 

 different from those now existing in the sea. One 

 suggestion of special interest that has been made 8 

 is that the concentration of carbon dioxide in the 

 atmosphere, and hence also in natural waters, was 

 very much greater than it is to-day. Free oxygen, 

 indeed, may have been entirely absent, and all the 

 free oxygen now present in the air may owe its exist- 

 ence to the subsequent splitting up of carbon dioxide 

 by the action of plant life. With such possibilities 

 of differences in the conditions in this and in so many 

 other directions, may we not be well satisfied if, for 

 the time, we can say that the formation of carbo- 

 hydrates and proteids has been brought within the 

 category of ordinary chemical operations, which can 

 occur without the previous existence of living sub- 

 stance ? 



To return once more, however, to the free-swimming, 

 autotrophic flagellate. In the early stages of its 

 history the loss caused by sinking, and so getting 

 below the influence of light and the possibility of 

 further growth, must have been enormous. We may 

 conceive a constant rain of dead and dying organisms 

 falling into the darker regions of the sea, and thus 

 a new field would be offered for the development of 

 any slight advantages which particular individuals 

 might possess. Under such conditions we may sup- 

 pose that the holozoic or animal mode of nutrition 

 first began in the absorption of one individual by 

 another one, with which it had chanced to come into 

 contact. If the one individual were more vigorous 

 and the other moribund we should designate the 

 process " feeding," and the additional energy obtained 

 from the food might well cause the individual to sur- 

 vive. If the two individuals which coalesced were 

 both sinking from loss of vigour, the combined energy 



reatise on Zoology," Part I., Introduction. London, 1909. 



Snyder, '"Lit.,' without Oxygen," Science Progress, vol. vi., 

 1912, p. 107. 



NO. 2761, VOL. I IO] 



of the two might make possible a return to the upper 

 water layers, where, under the influence of light, growth 

 and multiplication would proceed, and we should, 

 I suppose, designate the coalescence " conjugation," 

 or sexual fusion. 



Other individuals, again, sinking in shallow water, 

 would stick to solid objects on the sea-floor, while 

 the flagellum continued to vibrate. The current pro- 

 duced by the flagellum under these conditions would 

 draw towards the organism dead and disintegrating 

 remains of its fellows, and again we should have inges- 

 tion and animal nutrition. At this stage we witness 

 the definite passage from plant to animal life. A 

 further stage is seen when a cup-like depression to 

 receive the incoming particles of food is formed at 

 the base of the flagellum, to be followed still later by 

 a definite mouth. 



The transformation from the plant to the animal 

 mode of feeding can be seen in action by studying 

 actual organisms which exist to-day. In the course 

 of my work on the culture of plankton organisms 

 there has flourished in the flasks, on several occasions, 

 a small flagellate belonging to the group of chryso- 

 monads, which was first described by Wysotzky under 

 the name of Pedinella hexacostala, and to which I 

 directed the attention of Section D at the Cardiff Meeting 

 in 1920. The general form of Pedinella resembles that 

 of the common Vorticeila, but its size is much smaller. 

 The body, which is only about 5// in diameter, is 

 shaped like the bowl of a wine-glass, and from the 

 base of the bowl, which is the posterior end, a short, 

 stiff stalk extends. From the centre of the anterior 

 surface there arises a single long flagellum, surrounded 

 at a little distance by a circle of short, stiff, proto- 

 plasmic hairs. Arranged in an equatorial ring just 

 inside the body are six or eight brownish-green chromato- 

 phores or chloroplasts. In a healthy culture Pedinella 

 swims about freely by means of a spiral movement 

 of the flagellum, which functions as a tractor, the 

 stalk trailing behind. The chromatophores are large, 

 brightly coloured, and well developed, and the organism 

 is obviously nourishing itself after the manner of a 

 plant, like any other chrysomonad. But from time 

 to time a Pedinella will suddenly fix itself by the 

 point of the trailing stalk. The immediate effect of 

 this fixing is that a current of water, produced by the 

 still vibrating flagellum, streams towards the anterior 

 surface of the body, and small particles in the water, 

 such as bacteria, become caught up on the anterior 

 surface, the ring of fine stiff hairs surrounding the 

 base of the flagellum being doubtless of great assist- 

 ance in the capture of this food. One can clearly see 

 bacteria and small fragments of similar size engulfed 

 by the protoplasm of the anterior face of the Pedinella 

 and taken into the body. The organism is now feeding 

 as an animal. In some of the cultures in which bacteria 

 were very plentiful nearly all the Pedinella remained 

 fixed and fed in the animal way, and when this was 

 so the chromatophores had almost disappeared, though 

 they could still be seen as minute dark dots. We can, 

 as it were, in this one organism, see the transition 

 from plant to animal brought about by the simple 

 process of the freely swimming form becoming fixed. 



In the group of dinoflagellates, also — the group to 

 which the naked and armoured peridinians belong — 



