362 FCETAL AND LARVAL DEVELOPMENT. 



towards the preservation of ancestral characters, in that larvae 

 are necessarily compelled at all stages of their growth to retain 

 in a functional state such systems of organs, at any rate, as are 

 essential for a free and independent existence. It thus comes 

 about that, in spite of the many causes tending to produce 

 secondary changes in larvae, there is always a better chance of 

 larvae repeating, in an unabbreviated form, their ancestral history, 

 than is the case with embryos, which undergo their development 

 within the egg. 



It may be further noted as a fact which favours the relative 

 retention by larvae of ancestral characters, that a secondary 

 larval stage is less likely to be repeated in development than an 

 ancestral stage, because there is always a strong tendency for 

 the former, which is a secondarily intercalated link in the chain 

 of development, to drop out by the occurrence of a reversion to 

 the original type of development. 



The relative chances of the ancestral history being preserved 

 in the foetus or the larva may be summed up in the following 

 way : There is a greater chance of the ancestral history being 

 lost in forms which develop in the egg ; and of its being masked 

 in those which are hatched as larvae. 



The evidence from existing forms undoubtedly confirms the 

 a priori considerations just urged 1 . This is well shewn by a 

 study of the development of Echinodermata, Nemertea, Mollusca, 

 Crustacea, and Tunicata. The free larvae of the four first groups 

 are more similar amongst themselves than the embryos which 

 develop directly, and since this similarity cannot be supposed to 

 be due to the larvae having been modified by living under pre- 

 cisely similar conditions, it must be due to their retaining 

 common ancestral characters. In the case of the Tunicata the 

 free larvae retain much more completely than the embryos 

 certain characters such as the notochord, the cerebrospinal 

 canal, etc., which are known to be ancestral. 



1 It has long been known that land and freshwater forms develop without a 

 metamorphosis much more frequently than marine forms. This is probably to be 

 explained by the fact that there is not the same possibility of a land or freshwater 

 species extending itself over a wide area by the agency of free larvee, and there is, 

 therefore, much less advantage in the existence of such larvre ; while the fact of such 

 larvae being more liable to be preyed upon than eggs, which are either concealed, or 

 carried about by the parent, might render a larval stage absolutely disadvantageous. 



