ORGANIC FORM AND ARCHITECTURE 687 



ment, there seems good reason for regarding it as an incipient organ « 

 which in the course of evolution may come to be of survival-value. 

 To be included along with vestigial structures in the strict sense 

 are those which are present, though apparently functionUss, only in 

 the embryonic or larval stages. Thus the embryo whalebone whale 

 has two sets of teeth which never cut the gum, but entirely disap- 

 pear; and no use can be suggested for the posterior visceral clefts of 

 the embryos of reptiles, birds, and mammals. The reason for not 

 including a structure like the notochord has been already explained. 

 But as another case in point we may mention the minute fila- 

 mentous processes described by Boy den (191 8) in the gill-clefts of 

 the embryos of some reptiles, and of the fowl. They are too minute 

 to be of functional significance, but they appear to be homologous 

 with ^ill-filaments. 



Perhaps it makes for clearness to keep by themselves those organs 

 that are laid down in both sexes, but remain undeveloped in one. 

 Thus the development of the milk-glands is normally arrested in the 

 male mammal, probably because of the absence of the appropriate 

 hormonic stimulation, or because of the presence of some hormonic 

 inhibitent. There are cases, however, of some activation in the males, 

 soon after birth, at sexual maturity, and under pathological condi- 

 tions. Merriam gives a circumstantial account of male lactation in 

 a hare {Lepus hairdi) (Hay den's U.S. Geological Survey , vi, p. 666). 

 Similarly, but in a very different type, the first abdominal appen- 

 dages in a male lobster or crayfish are strongly developed and are 

 functional in reproduction, whereas in the female they are small, 

 slender, apparently quite useless, and very variable. Many examples 

 might be given; but the point seems clear, that organs much 

 reduced in connection with sex-dimorphism should be kept apart 

 from vestigial organs in the usual sense. 



To complete this attempt to define vestigial organs more pre- 

 cisely, we may exclude abnormal arrests of development, when some 

 structure, such as an eye, is spoilt in the making, and does not 

 pass beyond very rudimentary expression. It is also desirable to 

 exclude cases where the individual as a whole suifers marked 

 degeneration in the course of its hfe. Thus the small ganghon of an 

 Ascidian that lies between the inhalant and exhalant apertures is 

 all that is left to represent the brain and spinal cord of the free- 

 swimming larva; but it is hardly comparable to the vestigial gill 

 seen inside the spiracle of the skate. Apart from the fact that the 

 Ascidian ganglion shares in a degeneration which affects a great 

 part of the animal, it is not a functionless structure. Similarly we 

 should separate off the retrograded eyes of some cavernicolous 

 crustaceans and fishes, which are minute even in the young stages. 

 The retrogression cannot be seen in the individual lifetime, as in 

 the Ascidians, for the eyes begin badly. It is more than arrested 



