136 THE CAUSATION OF DISEASE. 



Now it is a very suggestive fact that the more nearly 

 identical the ante-partem E, the more alike are the children. 

 The nearest approach to identity occurs in the case of twins, 

 and these, as every one knows, are more like one another than 

 brothers and sisters not of twin birth; The germs, and -sperms 

 from which twins are evolved have sprung into being under 

 very similar conditions, and the E, moreover, of the embryos, as 

 determined by blood, is very similar. As regards quality 

 of blood, ' it is practically the same, but the quantity may, 

 of course, differ, as when the placentae are not of the same size ; 

 and as regards pressure, the effect upon the two children may 

 be very different. Thus, even in the case of twins, some 

 structural differences may be a priori postulated ; sometimes 

 the difference is considerable. Professor Lucas, whose work 

 on. Heredity, so far as it is a record of facts, is the most 

 important one yet published, quotes from Pierre Beielly : * 

 " Twins differ in expression, voice, writing, gesture, carriage, 

 and in many other respects. . . . The Siamese brothers 

 themselves — these more-than-twin beings — were not of the 

 same height, and the truly striking likeness of countenance 

 showed a marked difference on close study." ..." A similar 

 diversity is met with among the young of animals," continues 

 Dr. Lucas ; " the difference among the pups of the same litter, 

 even when one has taken care to cross the bitch with the 

 one dog is often prodigious." Herbert Spencer speaks in a 

 like strain : " Plants grown from seeds out of one pod, and 

 animals produced at one birth are not alike, and sometimes 

 differ considerably. In a litter of pigs or of kittens, we rarely 

 see uniformity of markings, and occasionally there are im- 

 portant structural contrasts. I have, myself, recently been 

 shown a litter of Newfoundland puppies, some of which had 

 four digits to their feet, while in others there was present 

 on each hind foot what is called the ' Dew claw,' a rudimentary 

 fifth digit."! 



Some diversity among offspring born at the same time is an 

 a priori necessity, as I have again and again insisted. No two 

 peas from the same pod, nor any two members of the same litter 



* " Traite de L'Heredite Naturelle." Par le Dr. Prosper Lucas, vol. i. p. 105. 

 f Herbert Spencer, " Principles of Biology," vol. i.. § 86. 



