Apparent Instances and Objections 163 
into another, giving rise to a dorsal groove the mechanical 
production of which is evident since it undoubtedly re- 
sults from the pressure of the preceding spiral upon the 
succeeding. Now ina still more recent geological period 
paleontological discoveries show that the descendants of 
these cephalopods with a tightly rolled up shell have begun 
to unroll, and have then the form of an Archimedes, 
spiral with broader turns which no longer touch one 
another. But the dorsal groove persists even in these half 
rolled up shells, a proof that the younger cephalopods 
have repeated hereditarily this character which was 
acquired by their ancestors.'*° 
This is certainly a most interesting example, but it 
has not quite the force of complete proof. For besides 
the objection, which we shall examine later, that the 
groove is formed in the non-living substance of the shell, 
it does not exclude the interpretation, though it be only 
verbal and without any real foundation, that it was not 
the rolling up of the spiral upon itself that produced the 
inherited groove, but rather that both the tight rolling 
up and the groove were selected and fixed independently 
of one another by natural selection. 
The influence of dry, hot climates upon the develop- 
ment of the horns of cattle and sheep is well known. If 
certain individuals of a certain breed of cattle are trans- 
ported from a wet, cold climate to a hot dry climate, the 
horns increase in length and circumference and the skin 
thickens. The following fact seems to prove that this 
acquired elongation of the horns is inheritable. A cow 
was transported from Algau in Bavaria where the climate 
is moist and cold, into the dryer and hotter steppes of 
1297 6 Dantec: Traité de Biologie. P. 296—297. 
