PACKARD. — INHERITANCE OF ACQUIRED CHARACTERS. 347 



pletely transmitted ; or at least the tendency to such transmission, if 

 latent in one generation, may appear in a succeeding one. And in the 

 earlier geological ages this principle may have been much more active 

 than at. present. The hypothesis seems to be a good working one to 

 account for phenomena which cannot be otherwise explained, and 

 should not iu consequence of adverse, though often very able and 

 candid criticism, be set aside. On the contrary, if as the result of 

 Weismann's criticisms it be only provisionally adopted, should it not 

 lead to further experiments in the laboratory, and to further and more 

 thorough studies of the metamorphosis of animals, with a view to 

 ascertain how far they are correlated with changes of habit and 

 function ? 



The lines of future investigation in this field appear to lie mainly in 

 four directions : — 



1. In the domain of comparative cytology. 



2. In the study of the life histories or metamorphoses of animals. 



3. In the further observation of the facts of heredity as observed 

 in the raising of plants, in the breeding of domestic animals, and that 

 of the different races of mankind, and especially by laboratory experi- 

 ments like those of Semper, Bert, and others, in changing the sur- 

 roundings of organisms. 



4. In the study of the gradual modification and specialization of 

 some organs in forms now extinct, with the degeneration and loss of 

 others, a subject so fully worked out by Professor Osborn as regards 

 the teeth of mammals. 



III. Inheritance at corresponding Periods of Life. 



But perhaps the crucial cases will be found to occur in animals with 

 a complicated metamorphosis, because in such instances we can draw 

 the line between characters which are congenital and those which are 

 acquired. As regards the characters which appear in post-embryonic 

 life, it is not difficult to see that they have originated in response to 

 stimuli brought about by changes in the environment. 



My attention has been turned to this subject while studying the 

 complicated life histories of some of the Bomhycine moths, in which 

 there are usually five distinct larval stages, and sometimes as many as 

 nine, not to mention the pupal and imaginal stages. Now in each 

 and all of these stages the organism is as a rule adapted to some 

 more or less temporary change in its environment. It seems, the 

 more closely we observe the habits of some of these caterpillars, 

 almost capable of demonstration that the different temporary colors, 



