NEOLAMARCKISM 387 



due to an acceleration of growth in the mature 

 forms, and a retardation in the senile forms. 



In a memoir on the " Biological Relations of the 

 Jurassic Ammonites," * he assigns the causes of the 

 progressive changes in these forms, the origination 

 of new genera, and the production of young, ma- 

 ture, and senile forms to " the favorable nature of 

 the physical surroundings, primarily producing char- 

 acteristic changes which become perpetuated and 

 increased by inheritance within the group." 



The study of the modifications of the tertiary 

 forms of Planorbis at Steinheim, begun by Hilgen- 

 dorf, led among others (nine in all) to the following 

 conclusions : 



" First, that the unsymmetrical spiral forms of the 

 shells of these and of all the Mollusca probably re- 

 sulted from the action of the laws of heredity, modi- 

 fied by gravitation. 



" Second, that there are many characteristics in 

 these shells and in other groups, which are due solely 

 to the uniform action of the physical influence of the 

 immediate surroundings, varying with every change 

 of locality, but constant and uniform within each 

 locality. 



" Third, that the Darwinian law of Natural Selec- 

 tion does not explain these relations, but applies 

 only to the first stages in the establishment of the 

 differences between forms or species in the same 

 locality. That its office is to fix these in the organi- 

 zation and bring them within the reach of the laws 

 of heredity." 



These views we find reiterated in his later palaeon- 



* Abstract in Proceedings of the Boston Society of Natural History, 

 xvii., December 16, 1874. 



