INTRODUCTION 



It is customary to divide all living organisms into two great kingdoms, 

 animal and vegetable. A sharp boundary line between animal and 

 vegetable life can, however, be drawn only in the case of the more 

 highly developed organisms ; while in those of more simple organisa- 

 tion all distinctions disappear, and it becomes difficult to define the 

 exact limits of Botany and Zoology. This, in fact, could scarcely be 

 otherwise, as all the processes of life, in both the animal and vegetable 

 kingdoms, are dependent on the same substance, protoplasm. The 

 more elementary the organism, the more apparent the general quali- 

 ties of this protoplasm become, and hence the correspondence 

 between the lower organisms is specially striking. With more compli- 

 cated organisation, the specific differences increase, and the character- 

 istics distinguishing animal from vegetable life become more obvious. 

 For the present, it must be confessed, the recognition of an organism, 

 as an animal or a plant, is dependent upon its supposed correspondence 

 with an abstract idea of what a plant or animal should be, based 

 on certain fancied points of agreement between the members of each 

 class. A satisfactory basis for the separation of all living organisms 

 into the categories of animals or plants can only be obtained when it 

 is shown that all organisms distinguished as animals are in reality 

 genetically connected, and that a similar connection exists between 

 all plants. The method by which such evidence may be arrived at 

 has been indicated in the Theory of Evolution. 



From the palaeontological study of the imprints of fossil animals 

 and plants, it has been established that in former epochs forms of 

 life differing from those of the present age existed on the earth. It 

 is also generally assumed that all living animals and plants have 

 been derived from previously existing forms. 



The conclusion is a natural one, that those organisms possess- 

 ing almost exactly similar structures which have been united as 

 species under the same genera are in reality related to one another. 

 Indeed, it is permissible to take a further step, and assume that the 



B 



