1883.] 263 [Hyatt. 



development, a more indirect mode of development. The types 

 which are descended from these last have often a mode of develop- 

 ment which in many forms is an apparent return to the direct 

 mode of development again. 



The first two modes occur in the progressive series, the last can 

 occur only in the highly retrogressive or degraded forms and con- 

 sists of the following stages, to which naturallists acquainted 

 with the life histories of modern parasites will easily find 

 parallels. 



The degraded uncoiled forms of the Nautiloidea and Ammonoi- 

 dea, wherever they occur, whether in the Silurian or in the Creta- 

 ceous, invariably have close" coiled young, showing that they were 

 the offspring of close coiled or nautilian shells, that is of progres- 

 sive forms which have themselves been evolved from a series of 

 straight, arcuate, and gyroceran predecessors. Their uncoiling 

 then is a truly retrogressive character, and this tendency to retro- 

 gression is inherited in successive forms in several series. Their 

 whole structure is finally affected, the whorl is reduced in size, 

 and the complication of the sutures and shell at all stages of 

 growth is degraded, until in their development only the close 

 coiled young remain to testify to their exalted ancestry. In other 

 words the forms inherit the degraded characteristics at such an 

 early stage that it effects their whole life except the earliest 

 stages. If we examine any of the progressive series we find that 

 characteristic modifications or variations tend to appear first in 

 the adults, then in successive forms they appear at earlier stages, 

 and finally disappear altogether or become embryonic, and this is 

 the case also with the degraded characteristics, and doubtless 

 when carried far enough even the last fortress of the ancestral 

 characteristics, the larval stages would be invaded and the shell 

 become completely uncoiled and perfectly straight and cylindrical 

 from the earliest age. We have found specimens of Crioceras, 

 in which only a part of the first whorl was close coiled and the 

 embryo of the Baculite, the straight cone of the Cretaceous, and 

 Jurassic Ammonoidea still remains unknown. We have, there- 

 fore, in the life of a series heredity acting in such a manner that 

 new characteristics are being continually introduced into the adult 

 and adolescent stages to replace the ancestral ones which have dis- 

 appeared or been crowded back into the earlier or larval stages. 



