THE ARGUMENTS FROM EMBRYOLOGY. 463 



carved. If this piece of work is not promptly commenced 

 and pushed on fast, it will not be completed when the rest of 

 the house is ready: workmen and tools will still block it up 

 at a time when it should be available. Similarly among the 

 parts of an unfolding embryo, those in which there is a great 

 deal of constructive work must early take such shape as will 

 allow of this. Now of all the tissues the nervous tissue is 

 that which takes longest to repair when injured; and it 

 seems a not improbable inference that it is a tissue which is 

 slower in its histological development than others. If this be 

 so, we may see why, in the embryos of the higher vertebrates, 

 the central nervous system quickly grows large in comparison 

 to the other systems why by pre-adaptation the brain of a 

 chick develops in advance of other organs so much more than 

 the brain of a fish. 



I30d. Yet another complication has to be noted. From 

 the principle of economy, it seems inferable that decrease and 

 disappearance of organs which were useful in ancestral types 

 but have ceased to be useful, should take place uniformly; 

 but they do not. In the words of Mr. Adam Sedgwick, 

 " some ancestral organs persist in the embryo in a function- 

 less rudimentary (vestigial) condition and at the same time 

 without any reference to adult structures, while other an- 

 cestral organs have disappeared without leaving a trace." * 

 This anomaly is rendered more striking when joined with 

 the fact that some of the structures which remain con- 

 spicuous are relatively ancient, while some which have been 

 obliterated are relatively modern e.g., "gill slits [which date 

 back to the fish-ancestor], have been retained in embryology, 

 whereas other organs which have much more recently disap- 

 peared, e. g. teeth of birds, fore-limbs of snakes [dating back 

 to the reptile ancestor] , have been entirely lost." f Mr. Sedg- 

 wick ascribes these anomalies to the difference between larval 



* Studies from (he Morphological Laboratory in the University of Cam- 

 bridge, vol. vi, p. 84. f #*i P- 81. 



