272 
local progression in another direction; but general retrogression 
arises from impaired vitality, and is usually—at least so 
Ammonites shew—the prelude to extinction. But when retro- 
gression is local and arises say from disuse, while there is 
progression in other features more suitable to the organisms 
environment, a higher degree of development may thereby be 
obtained. It is reasonable to suppose that the more complex 
the organism the more likely is it to shew simultaneous 
progressive and retrogressive features. 
The laws which govern progression and retrogression 
among Ammonites are certain to hold good in evolution 
generally, although they may become more complex in relation 
to more complex organisms. Some illustrations of their 
operation I shall give presently. 
On THE LAWS GOVERNING THE TRANSMISSION OF 
VARIATIONS. 
From the facts given by Spencer* concerning the trans- 
mission of such abnormal Variations as supernumary digits, it 
may be gathered that there is no certainty in regard to their 
appearance in the offspring. They may not be transmitted at 
all; they may appear in the next generation in some members, 
and not in others; and they may skip a generation entirely, to 
reappear in some members of the next. Further, these varia- 
tions when transmitted are not confined to one sex, but may 
appear in either sex. 
Of pathological variations I have mentioned two classes— 
one due to injury, the other due to disease. Whether 
variations due to injury—mutilation for example—are inherited 
there is little evidence to shew. Darwinf has one or two 
remarks on this head, which are not altogether convincing. 
I may say that among Ammonites are many cases in which 
*« Principles of Biology,” p. 258 ; quoted from an Essay by Dr Struthers. 
“Descent of Man,” 2nd Ed., p. 60. 
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