THE BEGINNING OF LIFE. 221 



If the accordance of the evolution of families has 

 once been followed up to the Gastrula, we shall not pause 

 there, but must regard the similarity of the sperm cor- 

 puscules and germ cells from the Spongiadse to the Ver- 

 tebrata as a primordial common property, connecting 

 the animal and vegetal world; and prior to the acquisi- 

 tion of which, only those modes of reproduction took 

 place which have been maintained among Protista and in 

 heterogenesis. 



As the common basis of sexual reproduction in the 

 various families argues a common origin, asexual re- 

 production, directly connected as we have seen it to be 

 with sexual propagation, by means of unfecundated eggs 

 and germs, leads us constantly further towards the be- 

 ginning of life. But the cell furnished with a nucleus 

 and sheath is inseparable from the protoplasmic cor- 

 puscule devoid of nucleus or sheath, on the growth and 

 fission of which rests the reproduction of the lowest liv- 

 ing beings. 



Their origin from inorganic matter, as we have set 

 forth above, is a postulate of sound human understand- 

 ing. To this beginning we are led, not, as the opponents 

 of the doctrine of Descent are wont to say, by a dog- 

 matic after-philosophy, but by the unprejudiced consid- 

 eration and computation of the facts of individual devel- 

 opment."' 



