14 ELEMENTS OF PALEONTOLOGY 



evident that these are not sufficient for the understanding of the whole of the 

 evolutionary process. They are capable of explaining the transformation of 

 one existing form into one other form, but fail to account for the fact that 

 often two or more different forms have originated from a single ancestral type. 

 Isolation, biological or ecological separation, or habitudinal segregation, are 

 synonymous terms applied to a fourth factor, which is important for render- 

 ing the process of speciation more intelligible. These terms signify that 

 the descendants of one ancestral form living amid a definite set of ecological 

 (or environmental) conditions, begin to adapt themselves to different sets of 

 conditions, as a result of which they become ecologically separated or segregated. 

 Each group of descendants consequently becomes subject to different influences 

 of environment, and in responding to such develops along different lines, the 

 divergence becoming finally so great as to be of specific value. 



According to the view that has just been stated, it is necessary to 

 recognise that the whole process of evolution is a very complex one ; that it 

 is the ultimate outcome of a number of factors, each of which has its owii 

 special efficacy, and may be sometimes antagonistic to the others ; and that of 

 the various factors engaged the following four are the most potent and most 

 essential : variation, inheritance, natural selection and separation. This view is 

 perhaps to be regarded as the most satisfactory explanation of the organic 

 world and its upbuilding that has yet been 2:)ut forward. Nevertheless, though 

 it cannot be gainsaid that the four operative principles just mentioned are 

 actively at work, it is difficult sometimes to trace their causes. This is 

 particularly true of those factors known as variation and inheritance. As to 

 the former of these factors, the rival hypotheses of the Lamarck-Darwinian 

 and of the Weismannian school are contradictory with reference to the cause 

 of inheritable variation. With regard to the cause of inheritance, important 

 discoveries, such as the Mendelian law, have been made, but these are too far- 

 reaching to permit of a satisfactory account in limited space. 



Life-Period and Extinction of Species. — Observation shows that 

 different organisms are by no means equally susceptible to impulses received 

 from the outer world. Many fossil genera remain almost wholly unchanged 

 throughout a number of formations (Foraminifera, Cidaris, Nautilus, Lingula, 

 Terebratula, Insectivora),and hence may be designated as j^ersisfent or conservative 

 types, in contradistinction to variable types. The latter pass through rapid changes 

 at the beginning of their career, develop a great variety of forms, and send out 

 branches and off-shoots in all directions up to a certain point ; they may then die 

 out after a comparatively short period of ascendency (Nummulites, Graptolites, 

 Cystids, Blastoids, Tetracoralla, Perischoechinoida, Trilobitae, Eudistae, Ichthy o- 

 sauria, Pterosauria, Dinosauria, Amblypoda, Toxodontia, etc.), or in some cases 

 may even continue on to the present day with undiminished vitality (Spatangidae, 

 Clypeastridae, many land and fresh-water mollusks, crabs, lizards, snakes, 

 ruminants, apes). Not infrequently types that were originally variable pass 

 over gradually into persistent ; their power of adaptation dwindles, they grow 

 less plastic, become incapable of sending off new varieties, species or genera, 

 and as the less vigorous of their numl)er become worsted one after another, they 

 finally stand out as isolated relics of antiquity (Isocrinus, Hatteria, Tapirns, 

 Equus, etc.) in the midst of rehabilitated surroundings. A one-sided develop- 

 ment in a certain direction, excessive size, abnormal (hypertrophic) jDeculiarities, 

 or too high specialisation of organs, is as a rule injurious to the form and 



