24:0 THE AMERICAN MONTHLY [October, 



expect the descendants to spread as far from the starting point and 

 into as great a variety of conditions as would be consistent with 

 well-being. A separation would thus result and various descend- 

 ants would be considerably sundered. We know by many ex- 

 perimental studies in bacteriology, for instance, that lowly organ- 

 isms can change their characters in response to changed condi- 

 tions ; can acquire the power to live at ease where their ancestors 

 could not have lived. The limits in which this is possible are 

 uncertain. It is conceivable on biological principles that all the 

 protozoa are descended from some single ancestral forms having 

 started in the remote past. This notion is the working hypothesis 

 used to-day by every working biologist. vSome forms, we may 

 say perhaps y4^^a?(5rt;, coming under new conditions of competi- 

 tion with carnivorous foes survived because of the protection it 

 secured by a stony covering, and Difflugia, a new genus, survives 

 till to-day, descended from this stock. Other Ama'ba: found power 

 in the body to make its own covering, and Hyalsophc7iia and 

 Clathrulina are the modern descendants of these. vStill other 

 AmcvSce, we may suppose, specialized the pseudopodia as organs 

 for collecting food and in connection, for protection and support, 

 specialized skeletons of lime or horn. These formed starting 

 points for new lines of development, and the operations within 

 these causes of divergence are conceived as having formed species 

 and later, as these grew moi"e numerous and diverged among 

 themselves, genera ; so on into wider and wider groups. In 

 these developments some lines of descent would retain the primi- 

 tive peculiarities much more strikingly than others. The Lobosa 

 are such an order, while of the Heliozoa and Radiolaria, which 

 appear related, the former seem more primitive than the latter. 

 As a class the Rhizopoda are more primitive than the Infusoria, 

 for the latter have far more peculiar and highly ditlerentiated 

 structure. But the Infusoria can still be conceived of as more 

 highly developed descendants of an amoeboid form in which the 

 pseudopodia are specialized locomotor organs, and the body is 

 fixed in outline. 



In the same way the orders and genera of the class Infusoria can 

 be understood as descended from certain central points, the ances- 

 tral direct line being nearly represented by forms well known but 

 not indicated in our synopsis. jParamccium is far more prim- 

 itive than Stcntor^ and Stentor than Vorticelhi and Pyxicolta. 

 It thus far appears that we can think ot the phylum as a great 

 natural group and, if it is so, that a great deal of interesting work 

 presents itself to him who would attempt to unravel the gene- 

 alogy. This is the problem of modern biology, and full of fasci- 

 nation it proves to be to all who seek its solution, as many have 

 done, with some indications that a solution is not very far distant. 



It would be interesting now to consider the mechanism of evo- 

 lution, but space will not permit and we must t,urn to the ques- 

 tion whether the facts of classification can be interpreted uj^on 



