EECOGNITION-MAEKS 171 



beautiful markings of the tail-feathers, by means of white 

 spots or bands on all the feathers except the middle pair. 

 The result was that when the tail was expanded during 

 flight, it was seen to be marked very conspicuously by white 

 bands, sometimes across the middle of the tail, sometimes at 

 the end, sometimes with one band, sometimes with two or even 

 three, so that the species were easily distinguished by this one 

 character. But the chief peculiarity to be noticed was, that 

 these bands w^ere only seen during flight, the white markings 

 being quite invisible when the birds were at rest. The impor- 

 tance of this fact I did not see till many years later, when, 

 in connection with other similar facts, it gave a clue to their 

 meaning and purpose. 



Xow that we have learnt how rapid are the powers of in- 

 crease of all animals, and the extreme severity of the process 

 by which the population is kept down to a nearly fixed amount 

 by the annual destruction of all the less adapted ; and further, 

 when we know how all the higher animals roam about in 

 search of their daily food, we are able to understand how 

 vitally important it is for all such animals to be able to recog- 

 nise their own species from all others without fail and at con- 

 siderable distances. This is essential for several reasons. 

 The young and half-grown, if they have strayed away from 

 the flock or herd, need to rejoin them as soon as possible ; the 

 two sexes of the same species require to know each other in 

 the same way by unfailing marks whether they are approach- 

 ing from behind or from the front; while the separate por- 

 tions of flocks divided by the sudden attack of some enemy 

 need to come together again as soon as possible. But there is 

 a still more important use of these distinctive markings, since 

 they are almost if not quite essential to the production of neiv 

 species by adaptation to change of conditions, as will be shown 

 later on. 



I first gave a somewhat full account of this class of mark- 

 ings, with several characteristic illustrations, in my Darwin- 

 ism, in 1889; but I had briefly treated the subject in my 



