382 Comparative Anatomy — Its History, Aim, and Method 



teristic integumentary structures of the former three groups — horny 

 scales, feathers, and hair — have no counterparts in the latter two 

 groups. Further, the scale, feather, and hair are so very different in 

 structure that, taken by themselves, no safe conclusion would be 

 justified concerning the mutual relationships of reptiles, birds, and 

 mammals. The hair might be a simplified feather or the feather might 

 be an elaborated hair. Structures produced by the skin readily undergo 

 adaptive change in relation to various environmental conditions. In 

 contrast to the skin, the central nervous structures are highly conservative. 

 The dorsal neural tube, with its uniformly five-part brain from which 

 emerge 10 or 12 pairs of very definitely located cranial nerves, and its 

 spinal cord with segment ally arranged pairs of spinal nerves, is a con- 

 stant feature of all known vertebrates. The neural tube of a shark is 

 unquestionably the same organ, as that of a mammal. Those highly 

 characteristic structures, the notochord and pharyngeal pouches, while 

 less conservative than the neural tube, are nevertheless certainly 

 recognizable in all vertebrates. These three very unique and more or 

 less conservative things, taken together, give us a satisfactory guarantee 

 of the genetic homogeneity of vertebrates of the most different sorts. 



The reproductive system is more conservative than some others, 

 although much less so than the nervous system. All the evidence indi- 

 cates that birds were derived from reptiles. In spite of profound ana- 

 tomic differences, the bird has retained the reptilian method of repro- 

 duction. Mammals, too, have apparently come from reptiles, but the 

 old reptilian type of reproduction persisted long after hair, milk- 

 glands, and other mammalian characteristics had been acquired. At 

 the present time, in the Australian region, there are two mammals, 

 the duckbill (Ornithorhynchus) and the spiny anteater (Echidna), which 

 lay large eggs similar to reptilian eggs. 



The motor mechanism, muscles and skeleton, is readily adapted 

 to changes in environment and locomotor habits. Yet, in spite of the 

 extreme differences in the external form and manner of use of ordinary 

 land legs, the whale's flipper, the wing of pterodactyl, bird, and bat, 

 and the human arm, the basic structure remains the same. The skele- 

 tons of all vertebrates exhibit the same basic pattern. In vertebrates 

 having paired appendages of the pentadactyl type, the homologies of 

 parts, even down to the minor details of the skeleton, are clearly 

 recognizable. Adaptation of the skeleton to the animal's mode of living 

 is effected by modification of the form, relative size, and number of 

 its constituent elements, but the pattern is conservative (Fig. 300). 

 Muscles seem to be more susceptible to change than skeletal parts. 



Digestive and respiratory organs, kidneys, and the blood- 

 vascular system are relatively low in the scale of conservatism. Di- 



