8 
regarded the genus and species as fixed and static. Now 
adays; in the light of the theory of evolution, the study of 
gxuietics, and tin* discovery- of extensive geograpiueal var- 
iation, the modern concept of these categories is dynamic. 
Tims K. Mayr <lefiiies animal species as groups of actually 
or pottmtially iiitei'h'i^eeding natural populations, which 
are I'epi oductively isolat(al from other such groups. Tlic 
s])(‘cies, tlum. is recognis(cl as a natural unit, not merely a 
convenient category fm* classifying museum specimens. 
Tile g(Mius and higlnn- categories aic* based on the natural 
]dienomenon that sp<'ci('s are not exauily distinct from om‘ 
another but can be aTranged in smaller or larger groui’is 
seinirated by smaller or larger gaps. But how many siicli 
groups ai‘(' to imhiuhal in one genus or family is de- 
cided as a matter of conv{'nieiiC(' by taxonomists, and thev 
are therefore not natural units. Bor instance, in mosquitoes 
Ave find it convenient to have a lew large geiuu'a divided 
into numerous subgc'mu-a, so that each genus contains hun- 
dreds of species. Brol)ably (nich subgenus <;f mos(]iiitoes is 
e(piival(uit to a genus in another group of anima's. but 
they are both natural groupings. 
It is ginu'rally acce])ted now that amongst animals, 
species usually evolve gradually, by small (diaiiges in iso- 
lated impulations. If think of evolution (‘ontinuing all 
ihe time, it folloAVs that w(c should (and do) find in naturt' 
all gradations upwards froni a ])opulatio]i that has becni 
sufficiently isolated from the nearest members of the same 
species to develop a few small distiucti\'e charact(^i‘s. ami 
thus may be an incipient sp(^ci(‘s. No syst(un of names can 
ad(a|uately represent all tlu' com])licatt'tl intmuadationships 
found in nature. 
Th(' sjnitial isolation lu’edcal for species d('V('lopmeut 
retiiiires that tluua' shouh] be practically no interbrecfliug 
with relat(Hl po])ulations. This usualh' nnums soim* g(‘o- 
gi'aphical bari*i('r which, depending on tlu^ animal, may be 
for ('xample water, mountains, dt^sert. i:'c oi* a belt of 
rain forest. Wheri' populations hav(' bc(Ui isolated for a 
('Onsiderabl(‘ p(U*iod, tlu'y may hav(' dcNcdoped mai’kc'd dif- 
fei'ema^s. yet still b(' abh^ to iiiterbicsul if the bari’ier 
betAveeu tlnmi disa])p('ais. TIk'sc geographical race's are 
often (tailed subspecies and give'u a Latin name in the same 
manner as a species. F. E. Zeuner has ('stimated that a 
subspecies can evolve in 10,000 years and that a period of 
boo, 000 to 1,000,000 ye'ars is ref|uired for evolution of a 
good species. 
Zeuner studied the morphological characters of the 
groii]) of birdwiiig bhtt(‘rflies Ovn}tho]'ifp.ra . and 
two allied genera of the family Fapilionidae Avhich occur 
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