248 COMPARATIVE ANATOMY 



which we recognise to-day may have sprung from different 

 uni-cellular groups. In this connection it is interesting to note 

 that, although the Flagellata and Ciliata are distinct classes of 

 the Protozoa, and have developed along characteristic lines, 

 they both exhibit similar tendencies in evolution. In both we 

 find the successive steps of simple adhesion of equivalent cells, 

 followed by organic connection with a differentiation into 

 vegetative and reproductive, somatic and germ cells, the series 

 culminating in the one case in Volvox, and in the other in 

 Zoothamnium. Volvox, however, must be ranked as a plant ; 

 and it^ should be noticed that the life cycle of plants is not 

 so simple as that of animals. Plants retain the capacity of 

 reproducing themselves asexually, as Volvox does by means of 

 its parthenogonidia, and an alternation of asexual and sexual 

 generations has been established, the one succeeding the other 

 with great regularity. It is by no means certain that the 

 macrogonidia of Zoothamnium can develop without fertilisation 

 or, as it is said, parthenogenetically, but if they can the fact is 

 exceptional in the animal kingdom. Some few animals, it is 

 true, are capable of reproducing themselves parthenogenetically 

 by means of reproductive cells which do not require fertilisa- 

 tion, and in them there is a more or less regular alternation of 

 generations. But in the great majority there is no such alter- 

 nation but an unbroken succession of sexual generations. 



It must now be evident that although we attempt to draw 

 a sharp distinction between the Protozoa and the Metazoa, 

 defining the one group as uni-cellular the other as multi-cellular, 

 the existence of the two series which we have been considering 

 breaks down the barrier implied by our definitions. If a 

 Metazoon is a multi-cellular organism reproducing itself asexu- 

 ally by ova and spermatozoa, it would be hard to say why 

 Volvox (leaving its vegetable affinities out of the question) 

 should not be classed among the Metazoa. It is, as a matter 

 of fact, placed among the Protozoa because it is directly 

 connected through Eudorina and Pandorina with the uni- 

 cellular Flagellates, and is not similarly connected by transi- 

 tional forms with any higher animal or plant. So it is simply 

 classed amongst its nearest kinsmen. And it is obvious 

 that, if the theory of evolution is true, and if we could recover 

 all the animal forms which have become extinct, it would be 

 impossible to fix the point where the Protozoa ceased and the 



