i INTRODUCTION 29 



If it be asked why all animals have not exchanged the larval for 

 the embryonic type of development, considering the advantage which 

 the embryonic phase possesses, from the point of view of the safety 

 of the young organism, it must be pointed out that the larval form 

 of development offers compensating advantages from the point of 

 view of wide dispersal of the species. The balance between these two 

 alternatives seems to have been easily inclined one way or the other. 



It is therefore of the essence of Comparative Embryology to 

 separate the fundamental ancestral traits of development from the 

 superficial and secondary, and this is the task that has been patiently 

 pursued for the last thirty years. As Sedgwick has pointed out, its 

 results have been highly disappointing, and this has led many to doubt 

 the validity of the ancestral explanation of development. But the 

 reason for this disappointment is largely a human failing which will 

 lead to equal disappointment in any branch of science. This human 

 failing is the ardent desire to settle fundamental questions in a few 

 years. Obviously the most difficult pages of the embryonic record to 

 decipher would be the earliest, for these have suffered most secondary 

 modification, and yet it is precisely over such questions as the first 

 differentiations of the embryo, such as the formation of the primary 

 tissues or so-called " layers " of ectoderm, endoderm, and mesoderm, 

 that most of the divergences of opinion have arisen. 



When we allow the mind to contemplate the vast profusion of 

 living species at present in the world, each with its own peculiar life- 

 history, and then reflect how few are at all known, we can see at 

 once how small a clearing we have made in the forest of comparative 

 embryology, and how premature it is to abandon the hope of finding 

 a law underlying the likenesses and unlikenesses of the various 

 modes of development. Where, as in the case of Vertebrata, the 

 knowledge is more complete than in the case of other groups, the 

 recapitulation of the structure of the lower members of the group in 

 the young stages of the higher, is so plain as to be obvious to all. 

 When the knowledge of other groups becomes equally complete the 

 same thing will be obvious there also. 



Those who have abandoned Comparative Embryology for Experi- 

 mental Embryology have set themselves the task of finding out the 

 mechanism of the transformation of the apparently formless egg into 

 the differentiated adult. But here again the impatience with delay, 

 the determination to arrive at "basal" principles at once, will prepare 

 disappointment for the workers in this branch also. 



Thus we find, as already pointed out, that whilst Driesch arrives 

 at the conclusion that each kind of life-history owes its peculiarities 

 to a non-material entelechy but leaves the resemblances between 

 the life-histories utterly unexplained, Herbst arrives at the con- 

 clusion that in each stage of development a substance is found which 

 acts as a " stimulus " to cause the development to the next stage, 

 while Loeb on the other hand maintains that until the conception of 

 " stimulus " is utterly abandoned no real progress with the subject 



