THE ORIGIN OF SPECIES 365 



most of the Lizards, Snakes, and Turtles have the ventricle 

 partially divided into two chambers, while the more special- 

 ized Crocodiles and Alligators have a complete partition and 

 therefore a four-chambered heart. This is the condition in 

 all adult Birds and Mammals, but the significant fact is that, 

 in the development of the heart of the individual Bird and 

 Mammal, embryonic stages succeed each other which parallel 

 in a general though remarkable way this sequence from a 

 two-chambered to a four-chambered condition as exhibited 

 in the adults of the lower Vertebrates. (Figs. 91, 92.) 



Or take the development of the brain in the Vertebrate 

 series. Even in the human embryo the fundament of the 

 brain arises by simple transformations of the anterior end of 

 the neural tube, which at first are nearly indistinguishable 

 from the conditions which exist in the lowest Vertebrates. 

 Then the changes become progressively more complex along 

 lines broadly similar to those occurring from Fish to Mammal, 

 until finally the complex human brain is formed. (Figs. 

 104, 105.) 



The same picture is presented by a study of the develop- 

 ment of the excretory system, the reproductive system 

 (Fig. 97), the skull, and so on. One cannot avoid the fact 

 that the organs of higher animals pass through develop- 

 mental stages which correspond with the adult condition 

 of similar organs in lower forms. The correspondence is not 

 exact, to be sure, but it is not an exaggeration to say that 

 embryological development is parallel to that which ana- 

 tomical study leads us to expect. A knowledge of the 

 anatomy of an animal actually gives a sound basis of facts 

 from which to predict in broad outlines its embryological 

 development. (Fig. 191.) 



What are the bearings of these facts on the evolution 

 theory? It is perfectly logical to conclude that it is an 



