THE EVOLUTION OF PROTOZOA. 309 



There can be no doubt that large size is a feature with 

 survival-value, provided it is not carried to excess. In a single 

 cell possibilities are of course limited by the necessity of keeping 

 the ratio of surface to volume sufficient to allow of free diffusion 

 of oxygen in, and carbon-dioxide and other excretions out ; the 

 development of the system of the contractile vacuole to the 

 •extent it attains in some large forms {e.g. Amphileptus) is 

 possibly concerned with this necessity. But even so, a large 

 range of variation is possible. The advantage of large size in a 

 Protozoon is obvious to the practical biologist ; Coleps is swept 

 away by a current which Paramecium breasts without difficulty, 

 Euplotes is captured by the Heliozoon from which Bursaria 

 escapes. But, more independent and " fit " though the large 

 forms are, they have not attained their high estate by the 

 gradual extinction of smaller types. Both are equally vigorous. 

 Indeed, it is the same throughout the whole phylum ; the 

 extremely minute and the relatively gigantic, Bodo and Trache- 

 litis, rub shoulders, so to speak, in every environment. Natural 

 selection fails here, as before, in the explanation of the facts of 

 biology. 



5. Springing Organs of Halteria. 



This genus, the typical one of the family Halteridce, is 

 distinguished by the possession, in addition to its ciliary ring, of 

 .an array of stiff spines, varying in number in different species. 

 These spines are prolongations of the cuticle, and are movable 

 on their bases by contraction of themyophane layer immediately 

 underneath the cuticle. By sudden movement of the spines the 

 •organism can leap a good distance. 



The value of such an apparatus, in facilitating escape from 

 enemies, is obvious. Its sensitivity is very high ; a mere touch 

 of the tripod of the microscope, a current hardly visible, the 

 touch of a spine on a bit of debris, is sufficient to cause a violent 

 leap. But such structures as leaping spines would, if necessary 

 for the survival of one form, be equally necessary for that of 

 others of the same size and habits. If natural selection has 

 ucted here, should we not find in Coleps, Urocentrum, Enchelys, 

 .and the like, either similar structures or else others fulfilling 

 similar ends ? Should we not expect that all these forms, which 



