370 THE ZOOLOGIST. 



of their area, that the blending of forms is caused by the two 

 races commingling. Judging from insular experience I should 

 be inclined to believe that the theory of inter-breeding is begin- 

 ning at the wrong end, but rather that while the generalised forms 

 remain in the centre of distribution, we find the more decidedly 

 distinct species at the extremes of the range, caused not by inter- 

 breeding, but by differentiation. To illustrate this by the group 

 of the Blue Titmouse. We find in Central Russia, in the centre 

 of distribution of the family, the most generalised form, Parus 

 pleskii, partaking of the characters of the various species east, 

 west, and south. In the north-east and north it becomes differ- 

 entiated as P. cyaneus ; to the south-west and west into P. 

 cceruleus and its various sub-species, while a branch extending 

 due east has assumed the form of P. flavipectus, bearing traces of 

 affinity to its neighbour P. cyaneus in the north, which seems 

 evidently to have been derived from it. 



But the scope of field observation does not cease with 

 geographical distribution and modification of form. The closet 

 systematist is very apt to overlook or to take no count of habits, 

 voice, modification, and other features of life which have an im- 

 portant bearing on the modification of species. To take one 

 instance, the Short-toed Lark (Calandrella brachydactyla) is 

 spread over the countries bordering on the Mediterranean ; but, 

 along with it, in Andalusia alone is found another species, C. 

 bcetica, of a rather darker colour, and with the secondaries gene- 

 rally somewhat shorter. Without further knowledge than that 

 obtained from a comparison of skins, it might be put down as an 

 accidental variety. But the field naturalist soon recognises it as 

 a most distinct species. It has a different voice, a differently 

 shaped nest ; and, while the common species breeds in the plains, 

 this one always resorts to the hills. The Spanish shepherds on 

 the spot recognise their distinctness, and have a name for each 

 species. Take, again, the eastern form of the common Song 

 Thrush. The bird of North China, Tardus auritus, closely re- 

 sembles our familiar species, but is slightly larger, and there is a 

 minute difference in the wing formula. But the field naturalist 

 has ascertained that it lays eggs like those of the Missel Thrush, 

 and it is the only species closely allied to our bird which does not 

 lay eggs of a blue ground colour. The Hedge Accentor of Japan 

 (Accentor rubidus) is distinguished from our most familiar friend, 



