THE ARGUMENTS FROM EMBRYOLOGY. 371 



however, that these creatures could not secure the habitats 

 needful for them, without possessing during their larval 

 stages, eyes and swimming appendages which eventually 

 become useless ; that though, by losing these, their organiza- 

 tion retrogresses in one direction, it progresses in another 

 direction ; and that, therefore, they do not exhibit the need- 

 less development of a higher type on the way to a lower • 

 type. Nevertheless there are instances of a descent in 

 organization, following an apparently- superfluous ascent. 

 Mr Darwin says that in some genera of cirripedes, "the 

 larvoe become developed either into hermaphrodites having 

 the ordinary structure, or into what I have called comple- 

 mental males, and in the latter, the development has 

 assuredly been retrograde ; for the male is a mere sack, which 

 lives for a short time, and is destitute of mouth, stomach, 

 or other organ of importance, excepting for reproduc- 

 tion." 



§ 131. Comparative embryology shows us that besides 

 substitutions of organs, there are what may be called substi - 

 tuted modes of development. The same kind of structure 

 is not always produced in the same way; and some allied 

 groups of organisms have modes of evolution which appear 

 to be radically contrasted. The two modes are broadly dis- 

 tinguishable as the direct and the indirect. They may 

 severally characterize the general course of evolution as a 

 whole, and the course of evolution in particular organs. 



Thus in the immense majority of articulate animals, 

 metamorphoses, more or less marked and more or less 

 numerous, are passed through on the way to maturity. The 

 familiar transformations of insects show us how circuitous is 

 the route by which the embryo-form arrives at the adult form, 

 among some divisions of the Articidata. But there are 

 other divisions, as the lower Araclmida, in which the unfold- 

 ing of the egg into the adult takes place in the simplest 

 manner : the substance grows towards its appointed shape 



