LAW OF DEVELOPMENT KNOWN AS VON BAEE's LAW. 87 



due to the absorption of a larval or immature free stage into 

 embryonic life. 



Let us take an example. Let us try to picture to ourselves 

 the steps by which the tadpole stages of the frog might be lost, 

 so that the adult frog arose direct from the egg. The larval 

 organs of a tadpole cannot disappear one by one independently 

 of one another. If the gill slits disappeared before the heart 

 had become double and the lungs had developed, the tadpole 

 would die of asphyxia. In order to completely obliterate the 

 piscine stage from the tadpole, you require a number of 

 nicely co-ordinated variations affecting different organs in 

 very different ways — all tending to the atrophy of those 

 organs which adapt it to an aquatic life and to the 

 development of the organs required for terrestrial life. 

 Such a combination of suitable variations as is here re- 

 quired — such an inversion of the original evolutionary changes 

 — is very unlikely to occur,^ especially when the same object 

 can be obtained, namely the obliteration of the piscine phase 

 in the frog's life, by a simple single variation — that is to say, 

 by the mother becoming viviparous and retaining its young 

 within its uterus or oviduct until the piscine stage of develop- 

 ment has been passed through ; or by the ovarian ovum de- 

 veloping a greater amount of yolk, so that the whole develop- 

 ment up to the close of the piscine stage can take place before 

 hatching at the expense of the yolk. That larval stages do 

 disappear and embryonic stages arise in this way is shown by 

 the case of the viviparous salamander (Salamandra atra), in 

 which the gills, &c., are all dereloped but never used, the 

 animal being born without them. Here, therefore, is an actual 

 case in which the larval phase has disappeared by becoming 

 embryonic and therefore functionless, and therefore largely 

 removed from the direct action of natural selection ; once em- 



' It has been suggested to me here that this combination of variations must 

 have taken place in phylogeny, otherwise the terrestrial animal could not 

 have been evolved ; why not then in the larva ? To this I reply : there is no 

 necessity for the long and laboured changes to be gone over again in inverted 

 order in the case of the tadpole, because the object can be obtained by the 

 simple inclusion of the tadpole stage within the embryonic period. 



