336 



NATURE 



[November ii, 1920 



made good by a plus on the other, or that 

 desirable dominants may corroborate one 

 another. 



Sterility is such a puzzling phenomenon that 

 any suggestion from competent biologists is very 

 welcome. The authors point out the striking fact 

 that both inbreeding and outbreeding may land 

 the organism in sterility, and they suggest that 

 there may be two quite different kinds of diverse 

 origin. Inbreeding tends to sort out homogeneous 

 pure strains in a stock, and in this sifting the 

 ability to reproduce may be lost. On the other 

 hand, outbreeding may bring together two germ- 

 cells too incompatible in their chromosomal com- 

 plex to allow of the continuance of the germ-cell 

 lineage. Thus it may be that the number of 

 chromosomes in horse and ass is too discrepant 

 to allow of fertile progeny. 



We have to thank the authors for a valuable 

 monograph, embodying the results of many per- 

 sona) experiments and a critical utilisation of 

 material previously available in the work of 

 others. The book is marked at once by Independ- 

 ence and by scholarship. Of great interest to 

 many will be the application of the biological 

 results to the particular case of man. There is 

 a carefully selected .bibliography, which might 

 have included perhaps a useful work by Reib- 

 mayr, " Inzucht und Vermischung beim Men- 

 schen " (1897). 



Einstein's Exposition of Relativity. 



Relativity: The Special and the General Theory. 

 A Popular Exposition. By Prof. Albert Einstein. 

 Authorised translation by Dr. Robert VV. Law- 

 son. Pp. xiii+138. (London: Methuen and 

 Co., Ltd., 1920.) Price 55. net. 



A POPULAR exposition of the doctrine of 

 Relativity and what it implies : for this 

 the world has been crying since the astronomers 

 announced that the stars had proved it true. 

 Here is an excellent translation of Einstein's own 

 book ; we hasten to it to know the whole truth and 

 nothing but the truth. The reviewer on this occa- 

 sion should be the man in the street, the man who, 

 with thousands, has been asking, " What is Rela- 

 tivity? " "What is the matter with Euclid and with 

 Newton?" "What is this message from the 

 stars?" Whether it is possible for the prophet to 

 make his message clear to the multitude, only 

 history can prove. He must needs speak 

 largely in parables, in incomplete similes; 

 and he is subject, therefore, to inevitable 

 misunderstanding. 



NO. 2663, VOL. 106] 



The plain fact is that Einstein asks the world to 

 give up preconceptions, and to change its point of 

 view. Men jump at suggestions of the fourth 

 dimension, which promise some amusement, and 

 room for a play of fancy. But the fourth 

 dimension does not call for a very high flight of 

 imagination ; it is not taken very seriously ; it is 

 even recognised as a commonplace and somewhat 

 old-fashioned intellectual pastime. A much more 

 serious trouble comes in with the simple concrete 

 instance. A passenger sitting in a train finds 

 himself hurrying towards the other side of the 

 carriage. He remarks that the brakes have been 

 put on and the train is stopping. We are told 

 that he may also interpret his experience thus : 

 "The carriage remains permanently at rest . . . 

 during the period of application of the brakes, the 

 railway embankment, together with the earth, 

 moves non-uniformly in such a manner that their 

 original velocity in the backwards direction is con- 

 tinuously reduced." This seems to upset all the 

 mechanics we had ever learned. For while we had 

 always been carefully taught that the real explana- 

 tion of the phenomenon lies in the stopping of the 

 train while we ourselves move on uniformly, it is 

 now held to be indifferent whether we adopt 

 this attitude or think of the pressure of the 

 driver's hand on the brake lever as imparting a 

 change of motion to the whole earth and to our- 

 selves — to everything, in fact, except the train. 

 But the plain man feels there is a very real 

 difference, and the ardent mathematician must 

 indeed be living in an abstract world if he does 

 not feel it too. 



Here, indeed, is the great virtue of mathematics, 

 that in it one may escape from the tyranny of gross 

 perception. As Prof. Eddington puts it, the mathe- 

 matician is never so happy as when he does not 

 know what he is talking about. The engine-driver 

 applying the brake does not know what happens 

 either to the train or to the earth ; he does know 

 that he has some control over their relative motion. 

 The mathematician does not know, and does not 

 wish to know, what happens to this or that indi- 

 vidual thing. He takes that for granted. He asks 

 only what are the relations between events, or 

 rather what can the mind know about them. Here 

 only the genius can make progress, .^n unshackled 

 imagination and a keen logical sense must combine 

 in the adventure. The human mind has dared to 

 look upon all history in space and time as laid out 

 before it, and having thus overstepped the limits 

 of mortality, it must go back and ask how much 

 of this vision it may, as mortal, know. 



At this moment rain is falling, leaves are rust- 



