EMBRYOLOGY. 



record is almost always abbreviated in accordance with the 

 tendency of nature (to be explained on the principle of survival 

 of the fittest) to attain her ends by the easiest means. The time 

 and sequence of the development of parts is often modified, and 

 finally, secondary structural features make their appearance 

 to fit the embryo or larva for special conditions of existence. 

 When the life history of a form is fully known, the most difficult 

 part of his task is still before the scientific embryologist. Like 

 the scholar with his manuscript, the embryologist has by a 

 process of careful and critical examination to determine where 

 the gaps are present, to detect the later insertions, and to place 

 in order what has been misplaced. 



The aims of Comparative Embryology as restricted in the 

 present work are two-fold: (i) to form a basis for Phylogeny, 

 and (2) to form a basis for Organogeny or the origin and 

 evolution of organs. The justification for employing the results 

 of Comparative Embryology in the solution of the problems in 

 these two departments of science is to be found in the law above 

 enunciated, but the results have to be employed with the quali- 

 fications already hinted at ; and in both cases a knowledge of 

 Comparative Anatomy is a necessary prelude to their application. 



In accordance with the above objects Comparative Embryo- 

 logy may be divided into two departments. 



The scientific method employed in both of these departments 

 is that of comparison, and is in fact fundamentally the same as 

 the method of Comparative Anatomy. By this method it 

 becomes possible with greater or less certainty to distinguish 

 the secondary from the primary or ancestral embryonic characters, 

 to determine the relative value to be attached to the results of 

 isolated observations, and generally to construct a science out of 

 the rough mass of collected facts. It moreover enables each 

 observer to know to what points it is important to direct his 

 attention, and so prevents that simple accumulation of dis- 

 connected facts which is too apt to clog and hinder the advance 

 of the science it is intended to promote. 



In the department of Phylogeny the following are the more 

 important points aimed at. 



(i) To test how far Comparative Embryology brings to 

 light ancestral forms common to the whole of the Metazoa. 



