502 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



Finally the hinder limit in the other Vertebrates has been shifted 

 still farther back— one segment (Polypterus), three segments (Amniota) 

 or as many as five segments (Acipenser). 



As far forwards as the hinder limit of the palaeocranium there 

 is, as already indicated (p. 317), clear evidence that the cranial wall 

 represents a series of neural arches which have undergone fusion. 

 As indicated on the same page it is difficult to avoid extending this 

 homology to the mesotic portion of cranial wall lying still farther 

 forwards. As regards the prechordal portion of the cranium there 

 is no definite evidence, but if we regard the trabeculae as primi- 

 tively in continuity with the parachordals we have to grant the 

 possibility of even this part of the cranium being in series with 

 the portions farther back and therefore also originally vertebral in 

 constitution. 



In conclusion it must be remembered that the series of myo- 

 tomes is also continued into the head-region, and the occurrence of 

 typical myotomes as far forwards as the premandibular or oculo- 

 motor segment (p. 210) may be taken as strong evidence that the 

 segmentation of the mesoderm originally extended throughout the 

 head-region including its pre-chordal portion. 



(7) Embryology and the Evolution of the Vertebrate. — 

 The special charm and the chief importance of the study of em- 

 bryology reside in the fact that it is one of the main branches of 

 evolutionary science. The greater part of what is ordinarily called 

 evolutionary research deals with the possible methods and causes of 

 evolutionary change. The data of Embryology on the other hand 

 form a branch of synthetic evolutionary science which deals not with 

 possible causes or methods but with the actual facts of evolutionary 

 change, striving to map out the course along which this has pro- 

 ceeded. In compiling the record of evolutionary progress we are 

 dependent upon Comparative Anatomy and Palaeontology as well 

 as Embryology, and in formulating conclusions care has to be taken 

 that whenever possible they are based on the data of all three 

 sciences. In cases where these data are not in agreement care must 

 be taken to bear in mind the main disturbing factors which are 

 liable to invalidate the conclusions in each case. In reasoning from 

 Embryology and Comparative Anatomy the possibility that particular 

 features are modern adaptations to existence say within a uterus or 

 egg-shell or under any other set of conditions different from those 

 of the ancestor has to be borne in mind. In the case of Palaeon- 

 tology and Comparative Anatomy there exists the same danger of 

 error as besets the protozoologist when he endeavours to construct a 

 continuous life-history out of a number of isolated observations on 

 the dead animal — the error of arranging observations in a series 

 which is not natural, or on the other hand, if the seriation be done 

 correctly, of reversing its direction. In Palaeontology errors of this 

 type are peculiarly apt to arise on account of the extraordinary 

 imperfection of our knowledge. If a series of organisms a, I, c, d, 



