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 



