2 6o 



GENERAL BIOLOGY 



Fig. 159. The wing bones of a fowl, a, adult grouse ; 

 b, young dack (after Coues). 



flows on in the old channels in absence of obstructions 

 sufficient to turn it aside. And the channels are guarded 

 from outside influences by the protection that most living 

 species give to their young during a portion of their develop- 

 ment. We have 

 already seen in 

 both plant and ani- 

 mal series, how the 

 sex organs, espec- 

 ially the ovaries, 

 are developed with- 

 in the body, out of 

 harm's way. This 

 protection is ex- 

 tended to the em- 

 bryo. 

 How much more alike are the archegonia of bryophytes 

 and pteridophytes than any other parts these groups possess ! 

 Romanes' classical figures of vertebrate ontogeny, copied on 

 page 262, show how much more alike are the early embryos 

 of vertebrates than any of their subsequent stages. Where- 

 fore our systems of classification of organisms have tended 

 to be based more and more on developmental phenom- 

 ena. The bipeds and quadrupeds of old were merged as 

 mammals, animals that suckle their young; and the pri- 

 mary division of mammals became placentals and apla- 

 centals — those that nourish their young before birth through 

 the agency of a placenta and those which do not so. 

 Similar illustrations abound in all the higher groups. 



Palaeontology makes known to us the life of past ages, by 

 interpreting such fragments of organisms as have actually 

 come down to us. Embryology furnishes historical data of 

 a very different sort — not the organisms of the past, but of 

 the processes of the past, in so far as preserved in the 



