42 



PSYCHE. 



[Match 1S91. 



accumulated, it would become not only 

 inconvenient to repeat all the character- 

 istics of its ancestors, but it would be a 

 physical impossibility for ally individual 

 to reproduce them all in the same suc- 

 cession in which they had arisen ; life 

 would not be long enough nor vital pow- 

 ers strong enough to accomplish such a 

 process. Nature provides for such 

 emergencies by a law of replacement ; 

 and as stated above, when a part or 

 characteristic becomes useless, if it 

 stand in the way of the development of 

 other parts or other characteristics of the 

 same part, it is replaced to a greater or 

 less degree by the newer and more useful 

 modifications. This is the rule so far 

 as relates to an ordinary normal series 

 of forms when such a series can be 

 traced with abundant materials through 

 a sufficiently long period of geologic 

 time, as has been repeatedly shown by 

 Cope and one of the authors. Made 

 confident by such experiences we do 

 not hesitate to apply it to the insects 

 where positive evidence of this sort is 

 not yet forthcoming. 



If this be con:ect, it is evident for ex- 

 ample that the sucking-tube and other 

 correlative internal modifications origi- 

 nated in the pupal or adult stages of the 

 primitive Hemipteron, then became 

 fixed in the organization of the order, 

 and are now inherited at an early age, 

 having replaced or driven out the ances- 

 tral, primitive, perhaps Thysanuriform 

 mouth parts from the larval stage. The 

 assumption that the sucking mouth parts 

 originated in the pupal or adult stages 

 is considered probable, because, al- 



though there are many exceptions, char- 

 acteristics usually originate in the later 

 stages in other branches of the animal 

 kingdom. In Lepidoptera and Diptera, 

 which resemble the Hemiptera in hav- 

 ing the highly modified mouth parts 

 with a tubular arrangement, these char- 

 acteristic peculiarities are confined to 

 the later stages of development, and are 

 not found in their larvae. The larvae 

 of Hemiptera are also decidedly Thy- 

 sanuriform, and that they originated 

 from a modified Thysanuroid form 

 having biting mouth parts in the larvae 

 and sucking mouth parts in the later 

 stages, seems to be indicated by this 

 fact. We have already seen in such ex- 

 amples as the locusts, etc., that an earlier 

 development in the inheritance of the 

 characters of adults may effectually ob- 

 literate the Thysanuriform larva, and 

 in the Coleoptera, Neuroptera, etc., that 

 it is the earlier inheritance of the sec- 

 ondary larval characteristic which ac- 

 complishes this result. In- no case do 

 the pupal or adult characteristics become 

 accelerated in development so as to re- 

 place the larval stage in the second se- 

 ries of orders except in parasites such 

 as the parasitic Pupipara (ticks). The 

 young are in some of these species born 

 as pupae, and the ovarian and larval 

 stages are passed within the mother.* 

 As a rule, then, the orders having in- 



*Among tlie orders having the direct mode of develop- 

 ment a similar case to the Pupipara is to be found in 

 the plant-lice. These being viviparous, the young are 

 born in an advanced stage, and are in reality, although 

 wingless, comparable with active pupae. In the case 

 of the sexually perfect forms which emerge from 

 pseudova, they are, according to Comstock, in a still 

 more advanced condition. 



