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" This seems to be a reasonable deduction from what is observed 

 in phylogenetic series of Ammonites, where from the same stock 

 arise one series which continues to progress, another series which 

 retrogrades, though both lived together and were presumably sub- 

 ject to the same environment. 



" More marked still would be the effects if from any cause there 

 arose a difference among members of a species as to the time in their 

 lives when offspring were produced. There is the case in Man — 

 the professional classes defer marriage till late in life, agricultural 

 laborers marry very early. 



"These surmises illustrate what may be supposed to be accom- 

 plished in the differentiation of species by the transmission of de- 

 velopmental variations in accordance with the law of earlier inher- 

 itance. Further consideration will shew that, if some members of 

 a species acquire, on account of environment, habits necessitating 

 the increased use of one part, and other members acquire other 

 habits with different results, and so on, there would, in course of 

 time, arise from one original stock two or more species very differ- 

 ent from each other or to the parent form — simply because their 

 small initial differences had been constantly increased by the action 

 of the law of earlier inheritance." 



IV. Descriptive Terms.* 



Before attempting to enter upon the descriptive part of this essay 

 it is essential to define, as briefly as possible, the meaning of the 

 terms which are constantly employed in the descriptions of the 

 different forms. The term "coil" has been applied solely to the 

 whole shell, while "whorl " and "volution" have been used when 

 in the singular or when numbered only for a particular whorl or 

 volution. Thus the first whorl or first volution is the first completed 

 revolution of the shell, and so on. I have also been obliged to use 

 volution for parts of a single whorl in describing substages. 



In describing the aperture I have used the terms " crest " for pro- 

 jecting parts and " sinus " for inflections of the outline to distin- 

 guish them from the saddles and lobes of the sutures. The ventral 

 sinus of the aperture and lines of growth is here called the " hypo- 

 nomic sinus," it being due to the large size of the hyponome or 



* Special students of Cephalopoda will, it is thought, be grateful for this chapter. 

 Other classes of readers, if there be one who gets so far and has the courage to go farther, 

 can skip and refer to it in connection with the descriptions which follow. 



