138 ZOOLOGY 



and the processes of heredity and variation are essen- 

 tially the same in every case. We can actually reason 

 from a plant to an animal, as the experimenters of re- 

 cent years have so frequently shown. Professor Jen- 

 nings, working with minute Protozoa in ditch water, 

 can determine facts of the greatest importance for the 

 understanding of mankind. 



Then we have the fact that all life, so far as we know, 

 comes from preexisting life. How, when, or where life 

 originated we do not know. It may have had more 

 than one origin, but in any given case the presumption 

 that a particular animal or plant did not arise by 

 "spontaneous generation" is so. strong that we take the 

 fact for granted. In any event, it is impossible that 

 any of the higher forms should thus originate.- 

 Many 3- Not only is there this general uniformity in life 



organisms processes, but it is astonishing to note how few are the 

 tissues kinds of materials, or tissues, of which animals and 

 plants are constructed. The voluntary muscle fibers 

 of man, with their fine cross-lines like those on a file, 

 look like those of a beetle. The nerve tissue, connective 

 tissue, skin tissue, and so forth are substantially alike 

 in great numbers of different animals. So again in 

 plants, we find greenness always due to chlorophyll, 

 and the building material stiffening the walls of the 

 cells is cellulose. 



Homoiogy Then again, throughout long series of diverse types, 

 t ^ ie or & ans -> or working parts made of tissues, correspond 

 accurately. They are said to exhibit homology. No 

 one doubts that the eyes of a man, a dog, and a frog 

 represent the same structures ; although this is not true 

 of the eyes of an insect or a mollusk. The arms of a 

 man are homologous with the wings of a bird ; it is an 

 anatomical error, if a pleasing symbolism, to represent 

 angels with arms and birdlike wings. 



