1893.] ZOJ [Ryder. 



crude egg-burrow only, without subsequent parental care, as in the case 

 of Pityophis, was constructed. We can thus understand that the often 

 elaborate and intelligent nest-building habits of Aves were preceded 

 by the far cruder and hastier and simpler nesting habits of the Reptilia, 

 which, on account of the phylogenetic relations between the two groups, 

 should, on a priori grounds, be the case. 



The origin of the eggshell of the eggs of birds and reptiles may there- 

 fore be traced to physiological causes acting automatically under the con- 

 trol of those instincts or intelligent eftorts at self-preservation and 

 protection extended by the parent to the young even wiiile still in the form 

 of the outwardly and apparently quiescent condition of the egg. The pro- 

 longed retention of the eggs in the oviducts must have begun in reptiles 

 where the whole laying of a season is found to occupy the oviducts atone 

 time. Such prolonged retention would distinctly tend to develop a shell 

 owing to the operation of agencies that we can in a great measure trace 

 and specify as above. Such a retention of the ova within the oviduct for 

 a period would also distinctly tend to develop the amniote placental and 

 viviparous forms of development, provided the retention of the eggs was 

 from any cause prolonged. There is, in fact, much evidence to indicate 

 that eggshells or secondaiy egg envelopes were, in the first place, evolved 

 because of the prolonged retention of the eggs within the oviduct by the 

 wary female for purposes of protection. Such a prolonged retention of the 

 eggs in the oviduct was only the prelude to the evolution of placental 

 viviparity and to the highest forms of parental care as exemplified in the 

 human species. Both processes were, therefore, adaptive as they were 

 also manifestly superposed in the order of their development. The 

 mechanical genesis of the amnion was begun in fishes, and was completed 

 amongst higher forms. Its conditions have been in part traced by the 

 present writer and Dr. T. W. Shore. In the same way the successive 

 steps of the evolution of the allantois may be traced. It may accordingly 

 be shown that the lines of demarcation between egg-laying and vivip- 

 arous vertebrates are in large measure arbitrary, and that if the evolution 

 of these processes be carefully studied, direct and obvious connections 

 can be established between both. Not only is this the fact, but there also 

 now exist sufficient data to establish upon a tolerably firm foundation the 

 doctrine that the various types of placentation are developed as the results 

 of direct mechanical and physiological adaptation. The evidence for this 

 appears quite as clear as that which has been adduced above in regard to 

 the dynamical method and mechanical conditions under which the form of 



