The Recapitulation Theory in Biology 21 



tations which are reflected in embryonic changes. As a result, 

 "the embryological parallelism is qualified by irregularities 

 that are mostly small, in many cases considerable, and occas- 

 ionally great." 26 



The irregularities in the law noted by Spencer may be briefly 

 enumerated: (1) The substitution of organs and the suppression 

 of organs in certain embryos as compared with others, the 

 first illustrated by the succession of changes in the aerating 

 appliances of the mammalian embryo, the second by the 

 suppression of the foetal teeth of whales; (2) retrograde 

 development, as in the loss of limbs and senses in adult par- 

 asites; (3) substituted modes of development, the two typical 

 modes being direct and indirect. In the former case the 

 development of the animal proceeds from the egg to the adult 

 condition without intermediate stages. In the latter a cir- 

 cuitous route is to be noted involving often many metamor- 

 phoses. The latter mode is that of the higher or more com- 

 plicated animals because it reflects the many changes incident 

 to the embryo from the long succession of unlike conditions 

 through which the group has passed. The direct mode is there- 

 fore the substituted mode among the higher animals, the cause 

 being a prolonged constancy of conditions inducing finally an 

 adjustment of all parts of the animal to the settled condition 

 of its life as a whole. Similar facts are to be noted among 

 particular organs as such. 



Subsequent to the publication of the first edition of the 

 "Principles of Biology" Spencer's views suffered a change of 

 emphasis. In later editions the condemnatory sentences re- 

 garding recapitulation are missing. He has found, in fact, 

 that von Baer's law, when carried out consistently in evolu- 

 tionary terms, implies recapitulation. 27 



"This conception," says Spencer, "of a tree, symbolizing 

 the relationships of types and a species derived from the same 

 root, has a concomitant conception. The implication is that 

 each organism, setting out from the simple nucleated cell, must 

 in the course of its development follow the line of the trunk, 

 some main branch, some sub-branch, some sub-sub-branch, etc., 

 •of this embryological tree; and so on till it reaches the ultimate 



« Vol. 1, 1st American ed., 1898, p. 368. 

 " See the first two laws of von Baer, p. 9. 



