8 THE EMBRYOLOGICAL METHOD. [introd. 



When the Theory of Evolution first gained a hearing it became 

 of the highest importance that it should be put to some test which 

 should shew whether it was true or not. In comparison with this 

 all other questions sank into insignificance. 



Now, the principle which has been called the Law of von Baer, 

 provided the means for such a test. By this principle it is affirmed 

 that the history of the individual represents the history of the 

 Species. If then it should be found that organisms in their de- 

 velopment pass through stages in which the} r resemble other forms, 

 this would be prima facie a reason for believing them to be geneti- 

 cally connected. The general truth of the Theory of Descent 

 might thus be tested by the facts of development. For this reason 

 the Study of Embryology superseded all others. It is now, of course, 

 generally admitted that the Theory has stood this test, and that 

 the facts of Embryology do support the Doctrine of Community of 

 Descent. 



But the claims of Embryology did not stop here. In addition 

 to the application of the method to the general Theory of Descent, 

 it has been sought to apply the facts of Embryology to solve 

 particular questions of the descent of particular forms. It has 

 been maintained that if it is true that the history of the individual 

 repeats the history of the Species, we may in the study of De- 

 velopment see not only that the various forms are related, but 

 also the exact lines of Descent of particular forms. In this way 

 Embryology was to provide us with the history of Evolution. 



The survey of the development of animals from this point of 

 view is now complete for most forms of life, and in all essential 

 points ; we are now therefore in a position to estimate its value. 

 It will, I think, before long be admitted that in this attempt to 

 extend the general proposition to particular questions of Descent 

 the embryological method has failed. The reason for this is 

 obvious. The principle of von Baer was never more than a 

 rough approximation to the truth and was never suited to the 

 solution of particular problems. It is curious to notice upon how 

 very slight a basis of evidence this widely received principle really 

 rests. It has been established almost entirely by inference and 

 it has been demonstrated by actual observation in scarcely a single 

 instance. 



For the stages through which a particular organism passes 

 in the course of its development are admissible as evidence of 

 its pedigree only when it shall have been proved as a general 

 truth that the development of individuals does follow the lines 

 on which the species developed. The proof, however, of this 

 general proposition does not rest on direct observation but on 

 the indirect evidence that particular organisms at certain stages 

 in their development resemble other organisms, and hence it 

 is assumed that they are descended from those forms. Thus the 

 truth of the general proposition is established by assuming it 



