RECAPITULATION. 



ONE of the commonest errors, or fallacies in science, 

 at any rate in biological science, is hasty generalisa- 

 tion. A statement which is observed to be true in particular 

 cases is frequently stated to be generally true, and the 

 generalisation is widely accepted and commonly taught 

 until some independent critic arises to point out that there 

 are important cases in which it does not hold. The 

 Haeckelian generalisation that individual development is 

 a recapitulation of ancestral development is an example of 

 this. It has been current in various formulae. Haeckel 

 called it the biogenetic law. Professor Milnes Marshall, I 

 believe, was responsible for its expression in the form that 

 each individual is compelled to climb its own genealogical 

 tree. The pre-Haeckelian statement due to Von Baer was 

 that animals resemble one another more in the earlier 

 stages of their development, and diverge in form as they 

 approach the perfect or adult condition. Mr. Adam 

 Sedgwick has lately attracted considerable attention by a 

 trenchant criticism of the generalisation in all these forms, 

 and has put forward conclusions which are incompatible 

 with it, and which I propose to consider in the course of 

 this article. But it is certain that the doctrine in question 

 has never been supposed by modern working biologists to 

 be literally and universally true. If it were so we should 

 be able to discover the complete evolutionary history of any 

 animal from an examination of its individual development, 

 which is notoriously not the case. In fact it is admitted at 

 the present time by every investigator of evolution that 

 whatever hopes may have been formerly entertained con- 

 cerning the revelations to be obtained from embryology, 

 we have not obtained, and cannot obtain, from a knowledge 

 of ontogeny or the development of the individual, anything 

 more than occasional indications of phylogenetic history, 

 or the modification of the series of Qfenerations. Our g^reat 

 English embryologist, years ago, in his classical text-book. 



