292 THE EVOLUTIONARY CAREER 



no trace of crustacean or echinoderm tectonics (beyond, of course, 

 the phase of the establishment of the three germ-layers) 

 justifies us in deducing an ancestry for the mammals that in- 

 cluded some animal living in water and breathing by means of 

 branchiae. 



The fact that many embryonic phases are strictly individualized 

 means that trivial characters are superposed upon structural ones. 

 (It is highly probable that this individualization in ontogeny is 

 universal and may be recognized given sufficient investigation.) 

 Thus birds' eggs are obviously strictly individualized by the 

 patterns and colours of the markings, nevertheless these markings 

 are trivial characters. 



, 97. ON THE MORPHOLOGICAL METHOD 



This has been indicated above : it depends on comparative- 

 structural investigations which must be controlled by physiological 

 studies. 



Thus the tendency in ontogenetic processes is to accelerate 

 embryogeny. The conservatism of developmental processes is 

 very great, nevertheless it is minimized. Gill (or branchial) clefts 

 — actual perforations of the pharyngeal wall — occur, of course, in 

 the ontogeny of fishes and amphibia and are accompanied by the 

 formation of vascular gill-organs. In the Reptiles and Birds the 

 clefts still appear in the development, but — since air-breathing 

 by the lungs comes about — the actual, vascularized gills do not 

 develop. In the higher mammals, however, the pharyngeal wall 

 is never really perforated and only branchial cul-de-sacs develop 

 and these only in so far as they are of tectonic significance in the 

 development of, say, the Eustachian tubes, the thymus gland, 

 and other organs. The development of the lungs is accelerated 

 and the modifications of the typical piscine, branchial anlagen are 

 directed more and more to those new organs that the ontogeny, 

 so to speak, anticipates. 



Recent research also indicates the influence of specific growth- 

 stimulating agencies (hormones) on the rate of development. 

 Further, this influence may be differential, possibly retarding 

 some, and accelerating other embryonic tectonic processes. Such 

 accelerating and retarding agencies may possibly be introduced 

 into the ovum, via the spermatozoon, by selective matings. And 



