12 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



cavities in their hearts : two auricles and two ven- 

 tricles ; but it is certain that, whatever was the animal 

 that was the nearest common ancestor to the two, it 

 had only one ventricle. The right and left ventricles 

 of the hearts of birds and mammals are, then, not 

 homologous in the sense of being homogenetic ; they 

 have been acquired independently by the two groups, 

 in consequence of certain physiological needs ; they 

 are the result of similar modifying forces, and are 

 homoplastic, but not homogenetic parts. 



DEVELOPMENT. 



The last point to which the student must be intro- 

 duced is one of the very greatest importance. If we 

 study the animal kingdom throughout, we find that, 

 starting from the simplest mass of protoplasm, 

 we are gradually led to the complex and elabo- 

 rate structural and functional arrangements which 

 are found in so highly organised an animal as 

 man himself. If, on the other hand, we study the 

 developmental history of a highly organised form, 

 we find that it starts from a simple mass of pro- 

 toplasm, the egg, or ovum, as this plastid is called ; 

 this cell gradually becomes more and more elaborated, 

 and takes on the more complex arrangement which 

 may be seen in its parent ; we observe, that is, that not 

 only are there a number of stages in the different re- 

 presentatives of the animal world, but that there are 

 also a number of stages in the structural history of 

 every individual ; and we may go yet a step farther, 

 and say that in a broad and general way there is a 

 complete parallelism between the two. 



The results of the investigations and considerations 

 which flow from a study of the facts here indicated 

 are best expressed in an aphorism, which may at once 

 be laid to heart, and which will be abundantly proved 

 by a study of development and comparative anatomy : 



