THE EVOLUTION OF PROTOZOA. 30o 



hypothesis that the case has been developed by natural selection 

 is in itself quite workable, it does not fit in with the fact that the 

 Thecolobosa are found to be living side by side with, under similar 

 conditions to, their unprotected brethren. As I have myself ob- 

 served, the bionomics of a Difflugia or an Arcella differ in no wise 

 from those of any species of naked amceba ; both classes feed on 

 vegetable debris and minute Protophyta such as chroococcacea and 

 diatoms; the reactions to external stimuli are similar ; it is true that 

 the shelled forms are very sluggish, but this is only to be expected. 

 Now since the conditions of life are so similar, if the shell (which, 

 it should be remembered, is simply a permanent exaggeration of 

 the stiff protective cuticle that any amceba can form around itself 

 in an unfavourable environment) has been formed by the selection 

 of variations (no matter how caused), we should expect to find 

 that the naked forms were either extinct or, at any rate, rare ; 

 since a shell, if necessary to a Difflugia for the prevention of its 

 extinction, is equally necessary to any other kind of amceba. 

 Instead, we find that the shelled forms are actually in a minority 

 both in number of species, and in number of individuals. 



It may, of course, be suggested that they are immigrants 

 from the sea, where in company with the naked forms it is 

 possible that they have been supplanted by the Eeticularia ; but 

 it is impossible to understand how such a creature as a calcareous 

 Foraminifer is any better placed than a chitinous-shelled amceba. 

 In fact, the chief reason for the use of chalk as a building 

 material seems to be environmental, due to the quantity of 

 calcium compounds in sea water, and is not a point with special 

 survival-value at all. The whole question, therefore, is, to say 

 the least, extremely difficult of solution by any hypothesis based 

 on the assumption of the action of natural selection. 



2. Locomotion in Flagellata. 



While many Flagellata possess only one flagellura, this 

 number is as frequently exceeded. Thus in the group Hetero- 

 mastigoda (which includes the well-known genera Bodo and 

 Volvox, amongst others), the number of flagella varies from one to 

 three. Again, some Euglenoidea have one long flagellum used 

 for locomotion, and another (the parafiagellum), usually shorter. 



