INTRODUCTION. 



at all stages of its existence are reproduced in its descendants at 

 corresponding stages. The second of these laws asserts that 

 offspring never exactly resemble their parents. By the common 

 action of these two principles continuous variation from a parent 

 type becomes a possibility, since every acquired variation has a 

 tendency to be inherited. 



The remarkable law of development enunciated above, which 

 has been extended, especially by the researches of Huxley 1 and 

 Kowalevsky, beyond the limits of the more or less artificial 

 groups created by naturalists, to the whole animal kingdom, is a 

 special case of the law of heredity. This law, interpreted in 

 accordance with the theory of descent, asserts that each organism 

 in the course of its individual ontogeny repeats the history of its 

 ancestral development. It may be stated in another way so as 

 to bring out its intimate connection with the laws of inheritance 

 and variation. Each organism 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 Com- 

 parative 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 

 development 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 resultant of a series of influences of which that of 

 heredity is only one. As a consequence of this, the embryo- 

 logical 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 



1 Huxley was the first to shew that the body of the Ccelenterata was formed of 

 two layers, and to identify these with the two primary germinal layers of the Verte- 

 brata. 



I 2 



