S08 THE ZOOLOGIST. 



Such probably was the process. In favour of the view that 

 selection was here the guiding force, it may be argued that all 

 the connecting links have disappeared, save one which has been 

 driven (presumably) to parasitism. This is true ; but we are 

 faced with the same overwhelming difficulty as before. The 

 Ciliata are descended from flagellates, and have acquired in the 

 process a tropho-motor apparatus vastly superior to that of their 

 ancestors. They live side by side with flagellates, under pre- 

 cisely similar conditions, with precisely similar reactions to the 

 ordinary stimuli of the environment ; their food is similar ;- 

 practically the sole bionomic difference is that the larger ciliates 

 can ingest much more at once (which would indeed make for an 

 even more speedy extinction of the much smaller flagellates). 

 Everything seems to point to the conclusion that the higher 

 types should have supplanted the lower simply in virtue of 

 increased size and activity, and yet nothing of the kind has 

 occurred. If development had in this instance been due to the' 

 selection of the in every way more "fit " ciliates, the flagellates 

 as a class should be extinct, or at most confined to a few 

 struggling groups ; instead they are ubiquitous and racially 

 vigorous. These facts, surely, absolutely preclude the ascrip- 

 tion of the developmental process in this case to natural selection, 



4. Size in Ciliata. 



This is, as is well known, very variable in the different 

 forms. For example, a species of Halteria is about l-1000th in. 

 long; at the other end of the scale are forms like Trachelitis 

 easily visible to the unassisted eye. The two very common 

 genera, Paramecium and Stylonychia, both reach a maximum 

 size of about l-70th in. The widely-diffused Coleps is l-500th in., 

 in length ; Spirostomum is said to be sometimes as much as- 

 l-10th of an inch. (Personally, I have never found quite such 

 large specimens.) Speaking generally, the peritrichous forms 

 do not contain individuals of Very large size, l-200th in. being a 

 fair average ; Stentor, however, forms a notable exception. The 

 other groups give us a mixed multitude of all sizes ; the maxi- 

 mum is reached in the Heterotricha (Kondylostoma, Bursaria r 

 Spirostomum) and the Holotricha (Trachelitis, Amphileptus) . 



