PEEFACE. ix 



5. These forms and their similar characteristics are not derived by direct inheritance 

 from the common ancestor, in which all the forms are necessarily similar and primitive, 

 but originate everywhere independently of hereditary influences in the different series, 

 and also in all formations independently of chronological or chorological distribution. 



6. This evolution of similar morphological changes in the forms of different genetic 

 series must be regarded as the similar reactions or efforts of a common organism in 

 direct response to similar generally distributed physical causes active in the same habitat, 

 and are therefore necessarily similar to each other, though in different genetic series. 



i As a whole, they may be said to express the general tendencies of modification, due to 

 the efforts of ihe common-radical and common organization while spreading in all direc- 

 tions and in different genetic lines to respond to similar physical causes, and meet their 

 requirements with suitable changes. They are, therefore, structural equivalents of each 

 other in different series, and functional equivalents of the general requirements of the 

 environment or habitat, or, in other words, purely physical selections. 



7. Morphological Difference. — Differentials are absent in the first members of series, 

 on first appearance in their descendants transient, but afterwards tend to become inva- 

 riable, or fixed in the stock or series, being perpetuated by direct inheritance in succes- 

 sive generation's, species, etc. They finally often disappear in the retrogressive or highest 

 and last occurring members of each series, or in aberrant forms when on the same level. 



8. They have no determinate mode of succession, but are usually more or less isolated 

 modifications, and arise first in individuals or varieties, but afterwards become characteristic 

 of species, and finally of the major part of the direct line in species, or descendent series. 



9. They are, therefore, strictly adaptive, variable characteristics, and not directed in 

 their occurrence or development by any more or less invariable law of successive modi- 

 fication, as are the morphological equivalents. We have failed in finding any differentials 

 of great importance whose prepotencc as hereditary characteristics could not be accounted 

 for by the law of use and disuse in connection with habits. The differentials of small 

 series, species, genera, and families, which we have not been able to analyze thoroughly, 

 may be due to the action and reaction of individual animals upon each other, or, in 

 other words, to natural selection. 



10. Differentials, therefore, can be separated from -other characteristics of the same 

 parts by careful observation and close analysis of their behavior in series, but cannot 

 be specifically predicted from the study of other series; whereas, morphological equiva- 

 lents can be predicted with the same certainty as the recurrence of cycles in physical 

 phenomena. Thus we can say of any new series of Nautiloids or Ammonoids, that, the 

 habitat remaining similar, they will, whenever or wherever found, tend to develop arcuate, 

 coiled, close-coiled, or discoidal and finally involute forms in progressive series, and 

 reverse this process in retrogressive series. 



11. Acceleration in Development. — All modifications and variations in progressive 

 series tend to appear first in the adolescent or adult stages of growth, and then to be 

 inherited in successive descendants at earlier and earlier stages according to the law of 

 acceleration, until they either become embryonic, or are crowded out of the organization, 

 and replaced in the development by characteristics of later origin. 



12. G-eratology. — Modifications which tend to appear in the old age of the individual 

 of progressive series correlate with the modifications taking place in pathological series 

 of all grades, and in geratologous and retrogressive forms of all kinds, however progres- 

 sive they may be in certain characteristics. Geratologous forms, therefore, show that 

 the development of retrogressive characters has been stimulated so as to take the place 

 of the hereditary progressive, thus either partially or completely replacing them. Partial 

 replacement is often accompanied by the early development of hereditary progressive 

 characteristics. 



