98 ORGANIC EVOLUTION 



development correspond to the conditions seen in the lower 

 vertebrates (Plate 38). There is a stage when the mam- 

 malian embryo has gill-slits like a fish, also a simple tubular 

 heart and a blood circulation much more fish-like than is the 

 adult mammalian circulation. This we interpret as a remi- 

 niscence of the time when the ancestors of the mammalia were 

 aquatic animals. Birds and reptiles show in their embry- 

 ology a similar stage resembling the fish in many important 

 regards. The frog and other terrestrial Amphibia are 

 actually aquatic in early life, their tadpoles being very 



fish-like (Fig. 20). 



In these different stages in the 



O 



embryology of an animal we read 



FIG. 20. Tadpole of salamander (Am- the history of its evolution from 



b/ystoma), magnified z\ times. 



simpler forms to its present state. 



We say that the development of the individual tends to 

 recapitulate the evolution of the race, and in studying 

 embryology from this standpoint we are studying the 

 racial history. 



Many examples of the interpretation of race histories 

 from the study of embryology might be given among both 

 plants and animals. I will give but one more, chosen from 

 the higher Crustacea. The Dccapoda, the highest group 

 of the Crustacea, includes among many others several forms 

 familiar to us all : the lobster, the crawfish, and the crab. 

 The lobster (Plate 39) has the posterior part of the b'tdy 

 long and well developed, using it in swimming, and by 

 its aid the lobster is able to leap through the water to con- 

 siderable distances. We call this portion of the body the 

 abdomen. It is filled with powerful muscles, and is divided 

 into seven parts, or segments, which move freely upon one 



