664 



DR. J. C. MOTTRAM OJf SECONDARY 



the males and females of which are entirely alike in every respect 

 apart from their sexual and reproductive organs. Many, in 

 describing the Song- Thrush, remark "sexes alike," forgetting 

 the male's song ; nevertheless, it is necessary in a consideration 

 of this kind to accept the classifications of others. 



Yulnerability to enemies is essentially a relative term. Un- 

 fortunately a classification of birds according to their vulnerability 

 to enemies has not been found. It follows "that this subject must 

 be considered in detail. 



Birds are subject to attack from the air by hawks, whilst 

 feeding in trees from carnivorous mammals and reptiles, and 

 when on the ground from all three. It follows that birds 

 entirely aerial are more free from enemies than are arboreal or 

 terrestrial ones, and birds entirely arboreal more free tha.ii 

 terrestrial. All birds, with very few exceptions, are to some 

 extent aerial. Sea-birds must be almost as free from enemies as 

 aerial birds, as there is little evidence that they are preyed upon 

 by any aquatic animals ; and of sea-birds, the oceanic ones must 

 be especially free from enemies, as liawks are not found far from 

 land. The following table, compiled from Finn's ' World's Birds ' 

 (see Table I.), sets out the families from this point of view, and 

 shows that birds relatively free from enemies present little 

 sexual diflterence, whereas those presenting great sexual differ- 

 ences are only to be found amongst terrestrial and arboreal 

 birds. 



Table I, 



Notes.— Compiled from F. Finn's ' The World's Birds.' 



2=Usually no sex difference, and includes tlie following descriptions : — " Hardly 



ever a sex difference," " seldom a sex difference," " vei'y slight or no sex 



difference," "small sex difference," "little s.d.," "rare s.d.," " ofteuest 



alike," " usually small s.d." 

 1 = Usually sex difference, and includes the following descriptions: — "Sexes 



different," " often sex difference." 

 Tlie ahove method of classifying secondary sex characters is used throughout the 



paper. ' - - 



{(Jonthrimd at foot of iie.vt pd'je.) 



