254 HABIT AND INTELLIGENCE. chap. 



it) by the fusion of the cells into a mass of homogeneous 

 and structureless but living sarcode, which is in fact 

 germinal matter.^ This sarcode mass swims about by 

 means of cilia on its surface, and appears to perceive light 

 and to avoid obstructions ; it almost exactly resembles one 

 of the Protozoa, or animalcules. Within the sarcode mass 

 the organs belonging to the organism at its next stage of 

 development are gradually formed ; and when they are 

 complete and ready for action, the sarcode is either 

 gradually absorbed, which is the most usual case, or else 

 cast off' in mass.^ This mode of development is indirect ; 

 the Echinoderms, and the marine Mollusca and lower 

 They be- Annulosa, begin their life in the likeness of Protozoa, 

 form of before they begin to transform themselves into their 

 Protozoa. ii;iature forms.^ 



The fact stated above, that the germs of the higher 

 forms resemble the mature states of the lower ones, 

 necessarily follows from the fact that the higher and the 

 lower forms alike are differentiated out of perfectly simple 

 germs, the higher ones being the most differentiated ; and 

 these truths hold good in direct and mdirect development 

 alike. But more than this is true in indirect develop- 

 ment. In the case of the aquatic Invertebrata just 

 described, development begins and makes some progress 

 Change of on a plan of low organic tj^e ; this plan is not con- 

 develop- tinned but changed, and development begins anew on a 

 ment. higher t}pe. Such change of plan during the course of 

 development is very common among animals. In the 

 cases just mentioned, the first plan of development is on 

 the type of the Protozoa, which stands, as it were, at the 

 base of the whole animal kingdom, and from which, on 

 the theory of the origin of species by develojjment, all 



1 Dr. "Wj'-sTlle Thomson says of the emliryos of the Echinoderms (star- 

 fish, sea-urchins, &c.) : "After impregnation of the egg, and complete 

 segmentation of the yelk, the whole germ-mass is resolved into an oval 

 ciliated animalcule, composed throughout, and consisting entirely, of 

 homogeneous structureless sarcode." (Natural History Review, July 186-3.) 

 I presume the "segmentation of the yelk" is an incipient formation of 

 cells. * See note at end of chapter. 



* Dr. "Wyville Thomson, in Natural History Review, October 1864. 



