DISTINCTION BETWEEN SPECIES AND VARIETIES. 39 



cnmstances. Thus there are many species of Moths and 

 Butterflies, which are only known apart by certain spots of 

 colour upon their wings ; hut these, being always found in the 

 same situation, and of the same hue, are justly regarded as suf- 

 ficient characters for the distinction of species. But on the 

 other hand, if there be any difference in the circumstances under 

 which the two animals are found, it is necessary to ascertain 

 how far the variation may be due to this cause. Thus, most 

 Birds undergo a change of plumage according to the season of 

 the year ; and until these changes are thoroughly known, we 

 are in danger of considering as distinct species what are really 

 identical. For if we were to compare one bird in its winter 

 dress, with another of the same species in its summer plumage, 

 we should often find the variations sufficiently great to induce 

 us to regard them as distinct species, if we did not trace their 

 respective histories. In regard to the birds which constantly 

 inhabit our own country, this has been accurately done ; but 

 there are many species that migrate to distant countries, either 

 to spend the winter in a warmer region, or the summer in a 

 cooler one ; and in regard to some of these, there is still much 

 doubt, certain birds being known in one country by one plum- 

 age, and again in the other by a different one, and sufficient 

 evidence, whether they are of the same or of different races, not 

 being always obtainable. Even between the northern and 

 southern parts of one extensive district, such as India, there 

 will be occasionally found such differences in the plumage of 

 their feathered inhabitants, as would lead to their being regarded 

 as distinct species, if there were not evidence to the contrary, 

 derived from the intermediate forms presented by the birds of 

 the intervening country. 



17. Thus it is seen, that the discrimination of species is 

 often a matter of the greatest difficulty to the Zoologist ; and 

 the sources of error which have been mentioned, are by no 

 means the only ones. "We have hitherto noticed only those 

 variations, which may be produced by the influence of external 

 conditions, or of a natural tendency to variation, such as is par- 

 ticularly evident in the domesticated races. But there are most 



