132 How Living Things Come to be What They Are 



the human embryo resembles a certain stage of the fish em- 

 bryo, of the chick embryo, of the turtle embryo. In a group 

 of classes having the same general plan of structure the 

 embryos pass through comparable stages. The individual 

 embryo of one class resembles the individual embryo of an- 

 other class up to a certain point, when divergences appear. 

 But at no time does the embryo of one class represent the 

 adult of another class. The bird and the rabbit develop as 

 does the fish up to a certain point. Past that point the bird 

 and the rabbit embryos pursue a different course, leaving 

 the fish embryo to develop into a fish. The bird and the 

 rabbit embryos, on the other hand, continue a common course 

 still further and diverge from each other later, one to become 

 a bird and the other a rabbit. 



The Meaning of Embryonic Structures 



The biogenetic law has had to be qualified as a result 

 of accumulated knowledge. We see now that it is impossi- 

 ble for the individual in its brief lifetime to recapitulate the 

 whole history of life upon earth. While it is true that the 

 individual passes from a one-celled stage to the adult stage 

 as did the whole race, according to the evolutionary doc- 

 trine, the process of development is necessarily so condensed 

 that we cannot expect a complete recapitulation. More im- 

 portant, however, is another set of considerations. Theo- 

 retically each ancestor in any genetic line must have repre- 

 sented a type of animal capable of living at large in its own 

 surroundings. In the development of the individual, es- 

 pecially in the case of the higher animals, the embryo does 

 not live an independent life. It must be adapted to the 

 particular conditions under which it develops. The fish, for 

 example, swims about freely, captures food and evades en- 

 emies more or less successfully. The embryo of a bird or 

 of a mammal does not do any of these things. The food 

 is supplied by the reserve in the t^g in the one case, by the 

 blood of the mother in the other case. The organism is 



