SCIENCE-GOSSIP. 



in 



between species occupying practically the same 

 tract of country may be brought about. 



Hibernating Habit Tending to Isolation. 



Other habits that result in the isolation of 

 closely allied species occurring in the same districts, 

 and having the same food-plants, are well-illus- 

 trated in our fauna. Toxocampa pastinum hibernates 

 as a larva ; its close ally, T. craccae, hibernates as 

 an egg; yet both feed on Vicia. Boarmia consortaria 

 hibernates as a pupa; B. roboraria hibernates as a 

 larva ; yet both species are still restricted to oak. 

 It is quite evident that, even should hybridization 

 take place between such species as these, the pro- 

 geny must die, so different are the conditions 

 under which the vital processes connected with the 

 early stages of the two species are respectively 

 carried on, and every entomologist knows how 

 fixed is the hibernating habit in nature for almost 

 every species. 



Doubtful Isolation. 

 Where closely allied species, such as Leucania 

 impura and L. straminea, occur at the same time 

 and on the same ground, and are only partially, 

 perhaps, specialized as to food-plant, it is possible 

 that, in their early stages of separation, they were 

 much more specialized as to habitat than they are 

 now ; for it is usually the case, as is evident in the 

 example here given, that one of the two species 

 (L. straminea) is strictly localized as to habitat, 

 whilst the other (L. impura) is of more general 

 distribution ; for whilst the former is strictly 

 confined to marshes, the latter occurs in meadows, 

 woodlands, in fact almost everywhere. It is 

 possible, too, that even these two species are 

 strictly specialized as to food-plant, L. straminea to 

 reed (Phragmites), and L. impura to Carex. I am 

 not sure whether these two species cannot easily 

 discriminate each other by what may be a true 

 recognition mark ; for, in the meadows bordering 

 reed beds, where both sometimes occur in profu- 

 sion, I can separate them at night as they are 

 flying, with the greatest ease, the white colour of 

 L. straminea making that species very conspicuous. 



Isolation by Diverse Habits. 

 In some cases, then, allied species may have 

 been differentiated from a common stock by the 

 development of diverse habits, as well as by a 

 difference of form or colour ; and the development 

 of different habit — necessitating, as it may do, 

 specialization with regard to a particular food- 

 plant, hibernating in a different stage, appearing 

 at a different time of the year, becoming single, 

 instead of double-brooded, or vice versa, etc.— is 

 almost sure to be correlated with a difference of 

 colour, form or structure, for it undergoes its 

 metamorphoses under different conditions, and we 

 know now that a difference in the nutritive value 



of the food given, and the formation of the wing 

 pigments under varying conditions, etc., are 

 accompanied by distinct changes of size or colour 

 in the imago. In this way species might readily 

 be formed on the ground where they actually 

 occur, and the same general rule would hold as to 

 both insular and continental areas ; and, in this 

 way, one can readily understand why in some 

 districts large numbers of closely allied species 

 maintain their distinctness. Even such a fixed 

 habit as flying between 8 p.m. and 9 p.m. is 

 sufficient to isolate a species from an ally flying 

 say between 7 p.m. and 8 p.m., and we know that 

 the flight and pairing of certain species takes place 

 regularly at a fixed time. In many species the 

 regularity of emergence from the pupa and of the 

 hour at which pairing takes place are so constant 

 as to cause experienced naturalists some surprise. 



Conditions for Specific Distinctness. 



I have been insisting on these particulars, 

 because it appears to me that the various 

 conditions laid down by our most eminent 

 naturalists for the formation of new species under 

 the influence of natural selection, are too narrowly 

 kept between certain limits. However important 

 (1) modification of structure of function, (2) the 

 development of recognition marks, and (3) the 

 necessity of some amount of infertility when 

 crossed with allied species may be, there are 

 other factors to be considered. The first of these 

 axioms is a necessity; for if the newly developed 

 form did not differ in structure or function from 

 the parent form, we could not recognize it as a 

 distinct species. There must be some modification 

 of structure or function, and this modification can 

 usually be correlated with a difference of habit, of 

 which indeed it may be the result. The study of 

 habits, then, is a useful addition to the work of the 

 biologist. It appears to me that "recognition 

 marks, and some amount of sterility when crossed 

 with allied forms," are useful adjuncts to, but not 

 necessities of, specific distinctness. 



Phylogenetic Age of Species. 



"We often speak of phylogenetically new and 

 phylogenetically old species, meaning thereby 

 species that have been more recently or more 

 distantly, in point of time, evolved from a parent 

 stock. If, however, we think for a moment, it 

 must be evident that each species traces back its 

 ancestry to the primeval ancestor of its class, and 

 the ancestry of all species of the same class is, in 

 point of time and from this point of view, equal. 

 Some species have, however, been evolved through 

 lines that have undergone more changes than 

 others, and hence some bear in their facies the 

 traces of a much more complex series of modifica- 

 tions than others. It is these remnants of bygone 



