372 THE EVOLUTION OF LIFE. 



by the shortest route. The Mollusca furnish contrasts which, 

 though less marked, are essentially of the same nature. Among 

 some Gasteropods, according to Vogt, the germ-mass, after 

 undergoing its earliest changes in the same way as germ- 

 masses in general, begins to transform itself bodily into the 

 finished structure : in one part, the component cells coalesce 

 to fonn the heart, in another part to form the liver, and so 

 on. But in other classes of molluscs, as the Cephalopods, 

 the e7ubryo is moulded out of the blastoderm, or superficial 

 layer of the germ-mass ; and the various organs, mostly aris 

 ing out of this blastoderm by a process of budding, reach 

 their ultimate shapes through successive modifications, while 

 they grow at the expense of the nutriment absorbed from 

 the rest of the germ- mass. And this indirect development 

 is universal among the Fertebrata. 



Now on contemplating in their ensemble, the facts thus 

 briefly indicated, we may trace among these irregularities 

 something like a general rule. The indirect development 

 characterizes the most-highly-organized forms. In the 

 sub-kingdom Vertebrata, which, considered as a whole, stands 

 far above the rest in complexity, the development is uniformly 

 indirect. It is indirect in the great mass of the Articulata. 

 It is indirect in the highest Mollusca. Conversely, it is direct 

 in a large proportion of the lower types. The eggs of 

 Protozoa, of Ccelenterata, of inferior Annuloida, originate the 

 respective structures proper to them, by transformations that 

 are almost immediate ; each of the cycle of forms passed 

 through, is assumed, when the proper time comes, in the 

 simplest way; and where they multiply by budding, the 

 substance of the bud passes by as short a process as may be, 

 into the finished form. Where among the simpler types of 

 animals, the evolution is indirect, its indirectness generally 

 appears to be related to some transitional mode of life, which 

 the larva passes through on its way to maturity ; and where 

 we find direct evolution among the more complex types, it ia 



