516 SCIENCE PROGRESS 



There are thus dissimilarities between plants and animals 

 which taken as a whole appear sufficient to constitute an essential 

 difference between the two phases of existence, a difference that 

 must necessarily extend to the primal substance of which they are 

 composed. If we cannot know whether or not there was unity 

 in the origin of the substance we need not for that reason be 

 deterred from concluding that there is duality in the develop- 

 ment, that is to say in the protoplasm at present extant in the 

 world. The fact that there are minute unicellular organisms 

 which appear composed of the same material and yet to be on 

 the border-line between the two categories of life, need not 

 embarrass us. These organisms stop short at the rudimentary 

 condition. They are rough sketches which are not elaborated 

 and are no obstacles to the view that the principle of life has 

 a dual manifestation. Throughout nature, in addition to well- 

 defined activities, there are to be found tendencies, overlappings, 

 rough models and abortive schemes which need not disturb the 

 judgment in the consideration of the finished work. It is the 

 indeterminate protista that have mainly given rise to the theory 

 of the unity of protoplasm, but these protista go no farther 

 than protista and should not give the rule for the well-defined 

 divisions that come after them. Even at the origin of life it 

 seems probable that the two phases must have been separate 

 unless we are to suppose that the one developed from the other 

 at some later period. But this is not a view to which it 

 seems possible to attach much weight. The motile spores of 

 algae can scarcely have passed out of the plant phase and become 

 the ancestors of animal existence. The amoeba which incorporate 

 their food and move by alternate contraction and extension of 

 their edges, together with all motile feeding micro-organisms 

 not undergoing transformation, might conveniently be considered 

 as animal and the few thousand temporary motile spores as 

 vegetal. 



Certain authors like Verworn frequently insist on the identity 

 of plant and animal life. It does not seem possible that there 

 should be identity when there are so many differences of habit 

 and of function. There is some reason to believe that the views 

 of the older investigators who saw an absolute division between 

 the two life states will be ultimately found to be less erroneous 

 than they have been held to be. 



It is not easy to find an exact parallel for the dualism which 



