COMPARATIVE ANATOMY 91 



of the lower, flowerless plants, such as the yeast plant, or 

 the Algcz, are very simple in comparison. In the same 

 way, among animals we find the lowly organized Amoeba 

 and its protozoan relatives, the more complex sponges and 

 jellyfishes, the still more developed flatworms, the annulated 

 worms, the Crustacea, the spiders, the insects, the Mollusca 

 (snails, clams, oysters, etc.), the starfishes, and the verte- 

 brates, including the fishes, Amphibia, lizards, birds, and 

 mammals. 



Now, what is the meaning of all this diversity of form 

 and the various degrees of complexity ? It is the theory of 

 evolution which interprets these phenomena, showing us 

 that the different degrees of resemblance and divergence 

 between these forms indicate different degrees of relationship. 

 Descent from common ancestors, with divergence under 

 the influence of natural selection and the other factors of 

 evolution, is the key to these phenomena. The taxonomic 

 system, or the system of classification of animals and plants 

 into varieties, species, genera, families, orders, subclasses, 

 classes, subkingdoms, and kingdoms, is but an expression 

 of relationships, the erection of a genealogical tree, in which 

 the animal and plant kingdoms would be the two great 

 branches, the lesser subdivisions corresponding to the 

 smaller branches and the twigs. The several species of 

 violets resemble one another because they are the descend- 

 ants of common ancestors, and that is what we mean when 

 we class them in the same genus Viola. Viola and Solea 

 in turn have a still more remote common ancestor, a fact 

 we express by placing the two genera in the same family, 

 the Violacecz. At some very much more remote period 

 the flowering plants were derived from the flowerless plants, 



