168 THE AMEKICAN MONTHLY [September, 



Until such genius arises or light breaks in from some other source, I say 

 again, I think it quite as logical and as satisfactory to keep to the old lines, 

 at least so far as to believe that, up to a certain stage in the progress of the 

 material world, there were no living beings, then they were created by an 

 almighty power, not expressed by condition and chemistry. I hold this 

 simply as a naturalist, for consistency's sake, and in order to go no farther 

 than the evidence warrants, so I am free to follow^ the lead of truth no matter 

 whither it may direct. 



The variety of types of the Protozoa is very great. This can scarcely be 

 appreciated except by long and intimate study. There is neither time nor 

 reason for an enumeration of these characters and peculiarities, although it 

 would be interesting to trace the advance in characters. As we proceeded from 

 the highest to the lowest of the groups we should find each type moi"e or less 

 intimately connected with those both above and below ; that is, the line of 

 phylogenetic descent is as clearly traceable in the protozoic as in the metazoic 

 branches of the animal kingdom. But this is not all, for we find certain In- 

 fusoria, for example, which are evidently the types connecting the origin of 

 the higher groups with the lower. We should also note, often with astonish- 

 ment, the remarkable capability of the disassociated, specific cell, and, by 

 the proper comparisons, find at every stage that the same functions or attri- 

 butes persist in the associated units of animal tissues. 



The Protozoa are separated into two grand divisions, Rhizopoda and Infu- 

 soria. The simplest of the former are naked, possibly reticulated protoplasm 

 only, nucleated and usually with a pulsating vacuole ; they lack all special- 

 ized organs of locomotion, prehension, or digestion, whilst the most highly 

 specialized Infusoria have their protoplasm surrounded by a firm, protecting, 

 and bounding wall, well defined, and often complexly differentiated aper- 

 tures for the reception of food ; their bodies have definite shape, and their 

 organs of locomotion are well developed. But from the lowest to the highest • 

 may be traced such plain biogenetic relations that the development of the 

 highest from lower is unmistakably revealed. Regard, for an example, the 

 sedentary Tentaculifera, the most highly developed ot the Infusoria. They give 

 birth to ciliated, free-swimming embryos, resembling closely the adults of 

 one of the three classes of the Ciliata which are less highlv organized. This 

 peculiar characteristic in the embr3'ology of the Tentaculifera seems to con- 

 clusively demonstrate their higher rank compared with the Ciliata. On the 

 other hand the adults are, without doubt, allied to the Metazoic Hydrozoa, 

 which also have ciliated embyros attesting their ascent from the Ciliata 

 through the Tentaculifera. So not only do the structural pecviliarities and 

 developmental phenomena of the unicellular animals plainly teach derivation 

 by biogenetic descent throughout the branch, but also indicate the starting' 

 point of various types of the Metozoa. In substantiation of this propositio'^' 

 it may not be amiss to point out examples in proof. Since the success ''^^ 

 of embryonic characters of the higher species appears to trace more or 

 certainly the ancestral or developmental history of that species, the cor "^S;" 

 ing stages of the two branches of animals are, in many cases, already ^^.7*" 

 lished. The larvse of the stai'-fishes and sea-uixhins are free-swimmin S kittle 

 bodies, surrounded by bands of cilia which unmistakably disclose th^' ^'^^^^" 

 tral affinities of the Echinoderms with the Peritrichous Ciliata. the Sj^f^ 

 Infusoria to which the well-known Vorticellse or bell animalcule?"' belong. 

 Another illustration may be mentioned. In the intestines of the common 

 frog and toad may at any time be found a flat, mouthless infusori.^" 1^^^'^'" 

 as Opalina ranartim ; it is covered throughout with fine, even ci'^'^* -tneie 

 hatches from the eggs of the Ccelenterata an animal not resemblinr^ "^^ pai ent, 

 but a cilia type, the planula, so closely resembling the parasite '^^'^^"^ the nog 



