INTEODUCTION 9 



tioii. Very ofteii a correspondence is observable with geologically older forms ; 

 and it is a well-known fact that all ammonites pass through early stages whicli 

 resemble, at least so far as chambering of the shell is concerned, Paleozoic 

 goniatites. A comparison of the inner whorls of an ammonite with its corre- 

 sponding goniatitic form, or with older ammonites, seldom fails to reveal 

 ties of kinship not otherwise discernible. Beecher has shown that nearly 

 every stage in the development of arm-supports in recent brachiopods corre- 

 sponds to conditions of the adult in some fossil genus ; and further, that the 

 chronological succession of the latter is to a certain degree identical with the 

 successive ontogenetic stages of recent forms. 



The relation of rudimentary or degradational organs occurring in recent forms 

 to those of the fossil ancestors of the latter is of extreme significance. By 

 rudimentary organs are meant certain structures (as, for example, limbs, 

 parts of limbs, organs of sense, respiration, digestion, reproduction, etc.), 

 which are still indicated by atrophied remains, but whose physiological 

 functions, and hence their Utility to the organism, have wholly disappeared. 

 Rudimentary organs are, as a rule, either normally developed in an embryonic 

 stage, or at least more strongly than in the adult individual, owing to a 

 process of degeneration, or retrogressive development. The fossil progenitors 

 of forms possessing vestigial structures are almost always characterised by 

 a füll development of the respective parts. The lateral metacarpals and 

 metatarsals in the horse and most ruminants, for example, are indicated only 

 by rudimentary side-splints ; but in an embryonic stage they are much more 

 strongly developed, and in related fossil forms they occur as normal bones, 

 carrying toes like the other metapodals, and serving for locomotion and 

 Support. The wrist and metacarpal bones in birds have also sufFered degenera- 

 tion, as is evident from a comparison with embryos and wdth older forms 

 (Archaeopteryx), which exhibit a much higher development. In like manner, 

 the teeth of birds have also become degenerated. In only a few forms 

 (parrots, ostriches) are faint dental ridges discernible during embryonic stages ; 

 but in all known Mesozoic birds the teeth are well developed and remain 

 functional throughout life. Similarly, teeth are developed during embryonic 

 stages in the baleen whale, but subsequently become atrophied ; while in the 

 older fossil Cetacea teeth are always present. Other instances of this 

 nature are to be met with in great profusion, both among vertebrates and 

 invertebrates. 



The biogenetic law is, however, not infrequently obscured, for the reason 

 that two closely related forms may not develop in exactly the same manner ; 

 embryos of the one type may be afFected by peculiar accelerating impulses 

 which are not shared by those of the other, and in consequence the first may 

 pass through certain stages very rapidly, or may even omit them altogether. 

 In this way the historical or palingenetic record contained in the development 

 of every individual may be to a large extent veiled, suppressed or rendered 

 unintelligible ; and this phenomenon of inexact parallelism (coenogenesis) is 

 especially common in highly differentiated types, where the embryo passes 

 through a multitude of phases. 



Paleontology and Phylogeny. — While conceding that by means of 

 embryological investigations zoologists and botanists are able to trace the 

 gradual development and differentiation of an organism through all its various 

 stages, and thereupon to construct a tree of descent (phylogeny) founded upon 



