ORGANIC EVOLUTION 255 



is a crustacean similar to those we had before us in 

 study 3 1 , but much more generalized in the possession of a 

 long series of similar swimming appendages, many of which 

 have in other malacostracans been modified into legs, 

 nippers, maxillipedes, opercula, stylets, etc., or altogether 

 atrophied (as in the abdomen of the crab, fig. 1586), with 

 good results. The horse-development series of figure 155 is 

 clearly a reduction series, and quite as clearly a series 

 illustrating the perfecting of the single hoof as an organ for 

 rapid locomotion on land. 



j. The correspondence between ontogeny and phylogeny. 



As phylogeny signifies the development of the race, so 

 ontogeny signifies the development of the individual. The 

 study of ontogeny is the special province of embryology, 

 and investigations in this field have brought to light, in 

 all the great groups of organisms, abounding examples 

 of likeness in plan of structure of developmental 

 stages of higher forms to that of adult organisms 

 lower in the same series. This we have already 

 noted in the salamander. It begins life as a single cell. 

 Its structure roughly corresponds to that of a protozoan. 

 It is, of course, as much a salamander and as little a proto- 

 zoan at that stage as it ever will be. But its plan of bodily 

 organization is very like that of a spherical single-celled 

 protozoan, and very unlike that of an adult salamander. 

 By segmentation, a blastula is formed, which is very like 

 Volvox in plan, since both consist of a hollow sphere of cells. 

 Then the process of gastrulation makes of it a gastrula, 

 which is much like a hydra, in that there are now two layers 

 of cells surrounding a simple food sac. The correspondence 

 between gastrula and hydra, it must be noted, extends only 

 to general body plan not at all to the specialized parts of 



