1899] CONSIDERATIONS CONCERNING SYMMETRY 107 
however, that anomalies are more common in man than in other 
mammals. In the latter anomalies of all kinds are rare. The pro- 
duction of a deformity, owing to some peculiar mental state, is not 
easy to follow out. There are very many factors at work. The 
mental and physical defects may be concomitant effects of the same 
cause, or the latter may be very remotely connected with the former. 
A deformity, if exposed, is, on the other hand, not necessarily asso- 
ciated with any aberrant mental condition. A structural change in 
the central nervous system may be associated with some distal change, 
but the distal change may be due to easily explained mechanical causes. 
If we revert to asymmetry in crystals, it will be recollected that 
attempts have been made to explain their asymmetry in their action 
on light, by referring to the asymmetric character of solar radiation. 
Some crystals rotate the plane of polarization to the right, others to 
the left, and two opposites are compared to a pair of gloves. The 
sun’s rays, passing south (as has been noted earlier in this paper), may 
be expected to produce effects on vegetable structures different from 
those produced by the north-going rays or the intermediate ones. The 
question of the effects of the sun-spots arises naturally. If these 
asymmetric rays and the portion of the solar surface exposed has 
favoured the growth of dicotyledons in one place, monocotyledons of 
great dimensions in another, and giant ferns in a third, what is to 
prevent our speculating on the changes that may have resulted from 
certain alterations in his demeanour in ancient times? Did the sun 
show less or more of one pole to the Silurian world? Was this 
followed by a bend that gave rise to the vegetable products of the 
carboniferous ? Was another change attended with the growth of the 
Triassic, and another with the growth of the Jurassic flora, until at last, 
after a tropical and cold period, the present temperate vegetation of 
the north, and the palms in the tropics and Dicksonias in the south, 
have been evoked by some new position of the solar globe ? 
In special breeds of domestic fowl abundant material can be 
obtained and the history can be studied. The sternum is often 
marked by a crooked keel, and the tail-bone and feathers are some- 
times wry. The bend of the keel is sometimes to the left and at 
other times to the right. A large number of specimens have been 
examined, but taking fifteen at random, there is a distinct bend to the 
left in nine keels and to the right in six. Tracing one of the best 
marked, the keel at the anterior part is seen to be a little bent to the 
right, followed back it leads to the left, crosses the middle line, forms 
a curve of considerable length, and, turning in to the median line, 
recrosses it to the right side. 
Two-thirds of the breeders consulted by me are of opinion that 
crooked sternum keels are hereditary, and that in-and-in breeding is 
accountable for the wry tails. 
One-third of the breeders consider the causes to be mainly 
