sect, ii.] INTRODUCTION. 9 



true in special cases, while its applicability to special cases rests 

 on its having been accepted as a general truth. 



Probably however the apologists of this method would main- 

 tain that the principle of von Baer, though its truth has not 

 been demonstrated directly, yet belongs to the class of " True 

 Hypotheses." To establish the truth of a hypothesis in a case 

 like the present in which the number of possible hypotheses is 

 not limited, it should at least be shewn that its application in all 

 known instances is so precise, so simple, and in such striking 

 accordance with ascertained facts, that its truth is felt to be 

 irresistible. 



Nothing like this can be said of the principle of von Baer. 

 Even if it be generally true that the development of a form is 

 a record of its descent, it has never been suggested that the record 

 is complete. 



Allowance must constantly be made for the omission of stages, 

 for the intercalation of stages, for degeneration, for the presence 

 of organs specially connected with larval or embryonic life, for 

 the interference of yolk and so forth. But what this allowance 

 should be and in what cases it should be made has never been 

 determined. 



More than this : closely allied forms often develop on totally 

 different plans ; for example, Balanoglossus Kowalevskii has an 

 opaque larva which creeps in the sand, while the other species of 

 the family have a transparent larva which swims at the surface of 

 the sea; the germinal layers of the Guinea-pig when compared with 

 those of the Rabbit are completely inverted, and so on. These are 

 not isolated cases, for examples of the same kind occur in almost 

 every group and in the development of nearly all the systems of 

 organs. When these things are so, who shall determine which de- 

 velopmental process is ancestral and which is due to secondary 

 change ? By what rules may secondary changes be recognized as 

 such ? Do transparent larvae swimming at the surface of the sea 

 reproduce the ancestral type or does the opaque larva creeping in 

 the mud shew us the primitive form ? Each investigator has 

 answered these questions in the manner which seemed best to 

 himself. 



There is no rule to guide us in these things and there is no 

 canon by which we may judge the worth of the evidence. It is 

 perhaps not too much to say that the main features of the de- 

 velopment of nearly every type of animal are now ascertained, 

 and on this knowledge elaborate and various tables of pliylogeny 

 have been constructed, each differing from the rest and all plau- 

 sible ; but it would be difficult to name a single case in which 

 the immediate pedigree of a species is actually known. 



The Embryological Method then has failed not for want of 

 knowledge of the visible facts of development but through ignor- 

 ance of the principles of Evolution. The principle of von Baer, 



