226 ANIMAL PEDIGREES 



directly connected with it, which tend to falsify the 

 ancestral history, many are now known, but time 

 will only permit me to notice the more important. 

 These are distortion, whether in time or space ; 

 sudden or violent metamorphosis ; a series of modi- 

 fications, due chiefly to mechanical causes, and which 

 may be spoken of as developmental conveniences ; 

 the important question of variability in development ; 

 and finally the great problem of degeneration. 



Concerning distortion in time, all embryologists 

 have noticed the tendency to anticipation or pre- 

 cocious development of characters which really 

 belong to a later stage in the pedigree. The early 

 attainment of the discoidal form in the shell of 

 Orbitolites complanata is a case in point ; and 

 Wilrtenberger has specially noticed this tendency 

 in Ammonites. Many early larvae show it markedly, 

 the explanation in this case being that it is 

 essential for them to hatch in a condition capable of 

 independent existence i.e., capable at any rate of 

 obtaining and digesting their own food. Ana- 

 chronisms, or actual reversal of the historical order 

 of development of organs or parts, occur frequently. 

 Thus the joint surfaces of bones acquire their 

 characteristic curvatures before movement of one 

 part on another is effected, and before even the 

 joint cavities are formed. 



Another good example is afforded by the develop- 

 ment of the mesenterial filaments in Alcyonarians. 

 Wilson has shown, in the case of Renilla, that in 

 the development of an embryo from the egg the six 

 endodermal filaments appear first, and the two long 



