INTRODUCTION. 3 



nection with the laws of inheritance and variation. Each organism 



o 



reproduces the variations inherited from all its ancestors at successive 

 stages in its individual ontogeny which correspond with those at which 

 the variations appeared in its ancestors. This mode of stating the law 

 shews that it is a necessary consequence of the law of inheritance. 

 The above considerations clearly bring out the fact that Comparative 

 Embryology has important bearings on Phylogeny, or the history of 

 the race or group, which constitutes one of the most important 

 branches of Zoology. 



Were it indeed the case that each organism contained in its de- 

 velopment a full record of its origin, the problems of Phylogeny would 

 be in a fair way towards solution. As it is, however, the law above 

 enunciated is, like all physical laws, the statement of what would 

 occur without interfering conditions. Such a state of things is not 

 found in nature, but development as it actually occurs is the re- 

 sultant of a series of influences of which that of heredity is only one. 

 As a consequence of this, the embryological record, as it is usually 

 presented to us, is both imperfect and misleading. It may be 

 compared to an ancient manuscript with many of the sheets lost, 

 others displaced, and with spurious passages interpolated by a later 

 hand. The embryological 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 ap- 

 pearance 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: (1) 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 qualifications 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 Embryology 

 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 



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