DIFFERENCES IN ANIMAL AND PLANT LIFE 515 



reigns. Self-fertilisation is largely to be found in plant life, but 

 is only to be met with among animals in some of the lower 

 forms. No doubt the reproductive process is very much the 

 same in plants and animals once fertilisation has taken place. 

 The agency of chance, however, plays a greater part in the one 

 than in the other. Since plants require the help of the wind 

 and of insects to convey the fertilising element and animals have 

 no such need, this fact constitutes a difference, and the difference 

 is accentuated when the selective characteristic in animals is 

 taken into account. The seeds of plants and animals are not 

 interchangeable. The pollen of a plant, it is needless to say, will 

 not develop in the ovary of an animal, and crosses between 

 distinct representatives of the two kingdoms are not obtained, 

 although no doubt it is not impossible to suppose that zoophytes 

 originally resulted from some accidental cellular fusion between 

 algae and marine animals. It is true that in the manner of cell- 

 division there is not a great apparent difference between plants 

 and animals. The attractive and repulsive forces at work in the 

 cell-field whereby the transformations are effected which result 

 in the splitting of the chromosomes are practically the same, as 

 far as staining reveals their working ; but the material on which 

 they work must necessarily be different. If it were not so it 

 seems evident there would not be dissimilarity of growth ; there 

 would only be one category of living things. 



The tissues differ in the two kingdoms. If a section be cut 

 from the stem of a higher plant and another from a typical 

 organ of the body of a higher animal and both be examined 

 under the microscope, it will be seen at once that a considerable 

 difference of structure exists. The cells in the former are 

 regular and separated by clearly defined cell-walls of definite 

 thickness, whereas in the latter they are irregular and almost 

 continuous. As Claus and other observers have shown, while 

 the plant cells retain their original and independent form, 

 sharply defined, those of animal tissue suffer numerous changes 

 at the cost of their independence and are often scarcely dis- 

 tinguishable in the mass of protoplasmic material, the reason for 

 this being that the plant cell is surrounded by a non-nitrogenous, 

 while the animal possesses a strongly nitrogenous boundary 

 wall of a far less definite character. The resemblance between 

 the two tissues is greatest in the lower forms of life. It becomes 

 gradually fainter as organic complexity increases. 



