IN MODERN SCIENCE. 



69 



US to believe that In some form it is necessary 

 in all. Haeckel's monistic view, however, re- 

 quires that in the lowest forms it should be ab- 

 sent and should have originated spontaneously, 

 though how does not seem to be very clear, as 

 the explanation given of it by him amounts to 

 little more than the statement that it must have 

 occurred. Still, as a " dualistic " process it is 

 very significant with reference to the monistic 

 theory. 



Much space is, of course, devoted to the tra- 

 cing of the special development or ontogenesis 

 of man, and to the illustration of the tact that 

 in the earlier stages of this development the 

 human embryo is scarcely distinguishable from 

 that of lower animals. We may, indeed, affirm 

 that all animals start from cells which, in so far 

 as we can see, are similar to each other, yet 

 which must include potent'ally the various prop- 

 erties of the animals which spring from them. 

 As we trace them onward in their development, 

 we see these differences manifesting themselves. 

 At first all pass, according to Haeckel, through a 

 stage which he calls the " gastrula," in which the 

 whole body is represented by a sort of sac, the 

 cavity of which is the stomach and the walls of 

 which consist of two layers of cells. It should 



