458 THE EVOLUTION OF LIFE. 



rently-superfluous ascent. Mr. Darwin says that in some 

 genera of cirri pedes, " the larvae become developed either 

 into hermaphrodites having the ordinary structure, or into 

 what I have called complemental 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 reproduction." 



§ 130fl. But now let us contemplate more closely the 

 energies at work in the unfolding embryo, or rather the 

 energies which the facts appear to imply. 



Whatever natures we ascribe to the hypothetical units 

 proper to each kind of organism, we must conclude that from 

 the beginning of embryonic development, they have a pro- 

 clivity towards the structure of that organism. Because of 

 their phylogenetic origin, they must tend towards the form of 

 the primitive type; but the superposed modifications, con- 

 flicting with their initial tendenc}^ must cause a swerving 

 towards each successively higher type. To take an illustra- 

 tion: — If in the germ-plasm out of which will come a verte- 

 brate animal there is a proclivity towards the primitive 

 piscine form, there must, if the germ-plasm is derived from a 

 mammal, be also from the outset a proclivity towards the 

 mammalian form. While the initial type tends continually 

 to establish itself the terminal t3^pe tends also to establish 

 itself. The intermediate structures must be influenced by 

 their conflict, as well as by the conflict of each with the pro- 

 clivities towards the amphibian and reptilian types. This 

 complication of tendencies is increased by the intervention of 

 several other factors. 



There is the factor of economy. An embr3'o in which the 

 transformations have absorbed the smallest amount of energy 

 and wasted the smallest amount of matter, will have an 

 advantage over embryos the transformations of which have 

 cost more in energy and matter : the young animal will set 



