448 AN INTRODUCTION TO ZOOLOGY 



tympanum, columella auris, etc., are added to it. Finally, in the 

 rabbit the process of improvement is carried still further, and we 

 have added an external auditory meatus, a pinna, a chain of ear bones. 

 Also to meet the much higher function of hearing we find a part of 

 the labyrinth specialised to form the cochlea, a very complex appa- 

 ratus for receiving and analysing sound vibrations. So that we can 

 actually trace the building up of the structures of the higher animals 

 through a successive series of stages in .lower forms. The reverse 

 is also true, for the complex branchial skeleton of Scyllium can be 

 traced through the stage in Rana up to that in the mammal, where 

 it has been reduced to the hyoid bone, and is utilised for a different 

 purpose from its original one. 



Yet another form of evidence is that supplied by what is known 

 as vestigial structures. We all of us possess muscles for moving our 

 ears and scalp, yet only a few are able to employ them, and even 

 then the movements serve no useful purpose. Why should such 

 structures be present at all ? They are handed down to us from 

 ancestors to whom they were useful. The appendix in man, the tiny 

 limbs in certain snake-like lizards, a small spur representing a hind 

 limb in Boa constrictor, the ligamentum arteriosum, and a multitude 

 of other structures are to be similarly explained. 



Embryological Evidence. 



In the portions dealing with embryology another variety 

 of evidence has been brought out. Thus, for example, we find 

 that the bird and the mammal pass through a stage of develop- 

 ment in which the heart, the blood-vessels, and the pharyngeal 

 region are in a condition resembling that found permanently in 

 the adult fish, so that the whole type of circulation is the same 

 and adapted to the aeration of the blood in the gills. Attention 

 was first called to facts of this sort in " Vestiges of the Natural 

 History of Creation," a book published in 1844 by Robert Chambers, 

 which appeared anonymously, since the author feared it would 

 damage his business. Observation of a large number of instances 

 similar to this soon led to the formulation of what has been termed 

 the Recapitulation Theory, or the Fundamental Law of Biogenesis. 

 According to this law, the reason for the appearance in the embryo 

 of conditions or structures that obtain permanently in the lower 

 groups is that each animal in its development recapitulates, or at 

 any rate hints at, the past development of the race. This has been 

 expressed in the aphorism, Ontogeny (the history of the individual) 

 reproduces Phylogeny (the history of the race). 



It is unnecessary to labour the point, however, that the record of 

 the race history kept in the developmental stages of any animal is 



