ox THE ORIGIN OF GENERA. 79 



ed back to an earlier period of life, then we have another persistent 

 type, but a new genus of a higher grade than its predecessor. 



In reviewing many examples everywhere coming under the eye 

 of the naturalist, it is easy to perceive what would constitute a 

 plastic and what a conserved condition of generic, or even of spe- 

 cific form. 



As one or more periods in the life of every species is character- 

 ized by a greater rapidity of development (or metamorphosis) than 

 the remainder, so in j^roportion to the approximation of such a 

 period to the epoch of maturity or reproduction, is the offspring 

 liable to variation. During the periods corresponding to those be- 

 tween the rapid metamorphoses the characters of the genus would 

 be preserved unaltered, though the period of change would be 

 ever approaching. 



Hence the transformation of genera may have been rapid and 

 abrupt, and the intervening periods of persistency very long ; for 

 it is ever true that the macrocosm is a parallel or repetition of the 

 microcosm in matter and mind. As the develoj^ment of the in- 

 dividual, so the development of the genus. We may add : so the 

 development of the whole of organized beings. 



These metamorphoses may be fitly compared to those in the 

 molecular constitution of matter. The force of cohesion between 

 the atoms of a vapor steadily increases with descending tempera- 

 ture, and in a regular ratio, till a given point is reached, when a 

 sudden metamorphosis to a denser or liquid condition takes place. 

 Nor have we reason to believe, with regard to many substances, 

 that there is any parallel relation between the temperature and the 

 molecular constitution before or after the metamorphosis takes 

 place. So, the temperature continuing to descend, the molecular 

 character of the liquid remains unchanged until, the vis conserva- 

 trix suddenly giving way at the ordained point, a soild is the re- 

 sult. Thus, while the change is really progressing, the external 

 features remain unchanged at other than those points, which may 

 be called expression-points. 



Now, the expression-point of a new generic type is reached 

 when its appearance in the adult falls so far prior to the period of 

 reproduction as to transmit it to the offspring and to their de- 

 scendants, until another expression-point of progress be reached. 



Thus a developmental succession does not so obliterate the lines 

 drawn around Nature's types as to render our system ineffectual 

 as an exDression of them. 



