82 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



either primitive ancestors or degenerate descendants. 

 But what is to be understood by degeneration ? 

 It must be defined as a loss of parts without a 

 corresponding development of other parts. From 

 this point of view all animals are degenerate on 

 some point or other ; mammals, for example, as 

 regards the weak development of the pineal gland 

 and of the coracoid bone. It may be affirmed that 

 there is degeneration only when the sum of the 

 subtractions is greater than that of the additions. 

 Embryology often furnishes us with interesting 

 data on the correct interpretation of rudimentary 

 and of vanished organs, and thus indicates to us 

 the phylogenic connections between various beings. 

 Without embryological studies we should probably 

 never have suspected that the Tunicates were 

 derived from primitive forms analogous to the 

 Vertebrates. But embryology has its limits, for 

 the transitory characteristics presented by the embryo 

 are but a partial reminiscence of the structural types 

 through which its ancestors have passed during the 

 geological ages. In addition, the characteristics 

 of the embryos are often but special adaptations 

 to the necessities of their embryonic life ; as, for 

 example, the allantoid and the placenta of the Verte- 

 brates. In a goodly number of cases the phylo- 

 geny can only be established and confirmed by the 

 discovery of the geological ancestors themselves. 

 It is the observation of real phylogenetic series 

 which demonstrates the existence or non-existence 

 of such^and such an intermediate type ; it allows 

 us to decide whether the rudimentary structures 

 represent organs in course of formation, or, on 



