304 THE ZOOLOGIST. 



It is real difficulties like these which are leading biologists to 

 believe that the part played by natural selection in evolution has^ 

 been in the past much over-rated ; that it acts rather as the 

 pruner of the Tree of Life than as its energy of growth ; and 

 that the final cause of evolution lies deeper, in the very nature 

 of life itself ; an idea as old as philosophy, and which has been 

 given its latest and most splendid expression in Bergson's doctrine 

 of the elan vital, the eternal self-evolving impetus which is Life 

 and Mind. 



The present article is an attempt to illustrate, by example 

 taken from the lowest phylum of animals, another difficulty 

 which is, I believe, more novel. This difficulty is, in brief, that 

 while there are many structures whose development is theoreti- 

 cally explainable (frequently easily so) by the theory of natural 

 selection, yet from the nature of present circumstances they 

 cannot have been developed by selection. This idea originally 

 suggested itself to me while engaged in the study of the Protozoa,- 

 and I have subsequently spent some time in its elaboration. 

 The main conclusions which I have reached are outlined in the 

 following pages. 



1. Pkotective Coverings of the Thecolobosa. 



The Thecolobosa are well known as amceb^ which surround 

 themselves with a protective case. Most are not of large size, 

 and there are not many genera ; the best known are Diffiugia 

 and Arcella, the former being commonest of all in this part of 

 the country (East Cornwall). The exact nature of the case with 

 which they surround themselves is not entirely clear ; it appears,, 

 however, to be chiefly chitinous, never calcareous or siliceous- 

 as in Eeticularia. It also appears that in one genus at least 

 {Diffiugia) the outer surface of the case remains sticky and 

 moist, and thus collects sand-particles and all kinds of debris,. 

 which cause it to much resemble that of a caddis-worm. 



It might be thought at first that here is an excellent instance 

 of selection ; the case clearly acts as a protection against enemies, 

 and by the contained organism's sealing up its mouth by a 

 temporary operculum, affords a safe asylum in time of drought. 

 This is perfectly true. But the difficulty is that while the- 



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