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 palaon- 
* Abstract in Proceedings of the Boston Society of Natural History, 
xvii., December 16, 1874. 
