LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 47 
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 developed 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 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. 
