LOCALIZED STAGES IN DEVELOPMENT. 165 



no difficulties in reasoning. Ecdysis removes all dark color from the body, 

 and this is redeveloped, and in development passes through stages which may 

 in a broad way simulate stages which we suppose may have existed in the 

 general evolution of color. The fact that in diverse orders of insects the same 

 conditions are found is not necessarily evidence for its phylogenetic interpre- 

 tation, but rather for the opposite, because the process of ecdysis is the same 

 in all insects, and like destructive and constructive stages and processes are 

 common to all. It is also true that the supposed phylogenetic stages are 

 products of our imagination, creatures brought into being for the purpose of 

 making observed conditions in ontogeny agree with cherished hypotheses. 

 Direct evidence upon this question is of course difficult to obtain, and it must, 

 at least for the present, remain one of opinion as far as its larger phases are 

 concerned. The smaller and more special phases, however, may perhaps be 

 more easily interpreted, and to these we shall next direct attention. 



On plate 17 are shown the succession of color patterns found in the larvae 

 of twelve species of Leptinotarsa. These figures represent well the different 

 types of species and color patterns found. In them we have clearly marked 

 "localized stages in development," and the question to be considered is 

 whether they are to be interpreted on the basis of atavism or on that of 

 physiology and development. 



On superficial examination it is strikingly apparent that in the lineata 

 group, for example, the larvae all have a similar generalized color pattern in 

 the first larval stage, that this is succeeded by one quite different in the second 

 stage, and in some species by still another in the third instar. When we trace 

 back the phytogeny of the genus we find that it goes directly to Zygogramma, 

 where many of the adult larvae have color patterns like those shown in the 

 second stage of so many Leptinotarsas. Back one step more in the ancestry 

 is Calligrapha, in which many of the adult larvae have spines and color pat- 

 terns like the young of Leptinotarsa. What further proof of biogenesis could 

 we desire? In the first instar of Leptinotarsa we have a Calligrapha condi- 

 tion, in the second a Zygogramma condition, and finally the pattern charac- 

 teristic of Leptinotarsa is attained. If we accept in all sincerity Haeckel's 

 dictum and apply it in the rigid manner of Cope, Hyatt, Jackson, and others, 

 the superficial evidence appears complete. 



In the face of evidence apparently so indubitable, I admit that the chances 

 that any other explanation will meet with favor are poor indeed ; yet I am 

 thoroughly convinced that these stages figured in the species of Leptinotarsa 

 on plate 17 are not to be explained upon the basis of atavism, but upon that of 

 the morphological and physiological constitution alone, and that the connec- 

 tion with ancestral or racial conditions is only a general one. Evidence in 

 support of this side of the question is not entirely wanting. 



