March 1S91.] 



PSYCHE. 



41 



the weevils among Coleoptera, and in 

 some whole orders, as in the Lepidop- 

 tera and Diptera, have been inherited 

 at snch early stages in accordance with 

 the law of acceleration in development 

 that they have replaced the useless Thy- 

 sanuriform stage. In other words, the 

 absence of this primitive larval stage in 

 the young of many specialized forms of 

 insects now living is due to the tendency 

 to earlier inheritance of the later ac- 

 quired, adaptive characters of the secon- 

 dary larval forms. 



It is very important for these consid- 

 erations to notice that after the insects 

 possessing the indirect modes of devel- 

 opment have passed through their re- 

 ductive secondary larval stages, they 

 return to the more normal or direct 

 mode of development in the pupa. In 

 doing this, they clearly illustrate the 

 exceptional and adaptive nature of their 

 deviations from the direct mode during 

 the larval stages, and show that this re- 

 sumption of the older beaten path 

 marked out by heredity is essential in 

 order that a typical hexapod form may 

 be evolved in the adult stage. The 

 pupa is always a six-legged form, with 

 the legs more or less developed, and 

 being common to all insects, whether 

 quiescent or active, is really a part of 

 the direct mode of development wher- 

 ever it occurs. It is as universal and 

 essential as are the typical ovarian and 

 adult stages. Indirect development is, 

 therefore, composite. It is first a devia- 

 tion in the larva from the direct mode, 

 and then a return in the pupa of the 

 direct mode, and this return necessarily 



brings the organism back again into the 

 normal line of evolutionary changes, 

 and the normal form of insect is the re- 

 sult of this return and the resumption 

 of progressive specialization. 



The reverse of this process, i. e. 

 when direct development is not re- 

 sumed, is shown in the case of parasites 

 like the female of Stylops. 



If it be true that the stages of develop- 

 ment in individuals are abbreviated rec- 

 ords of the modifications undergone by 

 the group during its evolution in time, 

 and that as a rule the characteristics of 

 adults of the more generalized or primi- 

 tive forms of any order, or even of 

 smaller divisions, in all groups of the 

 animal kingdom, show a tendency to 

 occur in the young of more specialized 

 forms of the same group or division, it 

 follows, that in each natural group the 

 specialized forms have been evolved 

 from the generalized forms. This ten- 

 dency to accelerate and abbreviate the 

 record preserved by heredity in the 

 growth and development of each indi- 

 vidual can be understood if one imagines 

 a series of forms evolving in time. First, 

 the representatives of the simple, primi- 

 tive ancestor ; then one form after 

 another coming into being successively 

 would each introduce some novel modi- 

 fications, according to its place in time 

 and the structural series. These modi- 

 fications being inherited at earlier stages 

 in descendants than those in which they 

 originated in the ancestral forms, would 

 crowd upon the characteristics already 

 fixed by heredity in the growth of the 

 young. By and by, as characteristics 



