IN THE ANIMAL KINGDOM. 75 



position to apply unreservedly any of the general principles 

 above stated, as hitherto no indications of sex have been 

 discovered in the majority of the species. Such points, 

 however, will be stated as seem to have any distinct bear- 

 ing on the subject before us. 



Under the name of Protozoa are comprehended certain 

 unicellular animals having more or less affinity \nth the 

 infusorial animalcules, in the restricted sense in which this 

 term is now used, after the elimination of the embryos of 

 the higher species, of unicellular plants, such as have been 

 already considered, and of other extraneous forms. But 

 there can be little doubt that names still occur in the list 

 of Protozoa, which do not represent real species, but only 

 the embryonic condition of such as are referable in their 

 adult state to far higher types of organization, for a trans- 

 formation which has now been satisfactorily traced in so 

 many instances, may fairly be suspected to occur in not a 

 few others still referred to the lower group. The detection 

 of such cases must be left to patient observation, with the 

 conviction that, though this is a slow process, it will even- 

 tually yield results which may confidently be relied on. 



But even in cases where long continued observation 

 shows that the unicellular organisms in question never at- 

 tain any liigher type of structure, there still remains be- 

 hind another difficulty — namely, to determine whether they 

 are of an animal or vegetable nature, and the grounds on 

 which this is to be decided, or the points of distinction be- 

 tween the Protophyta and the Protozoa, form a question 

 about wliich naturalists are not yet quite in agreement, 

 though sensibly approaching to it. The following are the 

 points now generally admitted as distinguishing the unicel- 

 lular animal from the vegetable : — 



1. Contractility of the substance or of the bounding 

 wall of the organism, which is a main agent in its locomo- 

 tion, though ciliary action is also employed, and to a much 



E 2 



