214 Animal Life and Intelligence. 



ceivable that the variation of one or more of the few cells 

 in early embryonic life may introduce a great number of 

 variations in the numerous derivative cells. In explana- 

 tion of my meaning, I will quote a paragraph from a paper 

 of Mr. E. B. Poulton's on " Theories of Heredity." * " It 

 appears," he says, "that, in some animals, the great 

 groups of cells are determined by the first division [of the 

 ovum in the process of cleavage f]; in others, the right 

 and left sides, or front and hind ends of the body ; while 

 the cells giving rise to the chief groups on each side would 

 then be separated at some later division. This is not 

 theory, but fact ; for Roux has recently shown that, if one 

 of the products of the first division of the egg of a frog be 

 destroyed with a hot needle, development is not necessarily 

 arrested, but, when it proceeds, leads to the formation of 

 an embryo from which either the right or the left side is 

 absent. When the first division takes place in another 

 direction, either the hind or the front half was absent from 

 the embryo which was afterwards produced. After the 

 next division, when four cells were present, destruction of 

 one produced an embryo in which one-fourth was absent." 

 Now, it is conceivable that a single modification or 

 variation of the primitive germ might give rise to many 

 correlated modifications or variations of the numerous cells 

 into which it develops ; just as an apparently trivial 

 incident in childhood or youth may modify the whole 

 course of a man's subsequent life. It is difficult, indeed, 

 to see how this could be effected ; to understand what could 

 be the nature of a modification of the germ which could 

 lead simultaneously to many favourable variations of bones, 

 muscles, blood-vessels, and nerves in different parts of the 

 body. This, however, is a question of the origin of varia- 

 tions ; and it is, at any rate, conceivable that, just as by 

 the extirpation with a hot needle of one cell of the cleaved 

 frog's ovum all the anterior part of the body should be 

 absent in development, so by the appropriate modification 

 of this one cell, or the germinal matter which produced it, 



* Midland Naturali$i, November, 1889. t See ante, p. 52. 



