单项选择题
Animals do not possess a language in the true sense of the word. In the higher vertebrates as also in insects, particularly in the socially living species of both great groups, every individual has a certain number of innate movements and sounds for expressing feelings, It has also innate ways of reacting to these signals whenever it sees or hears them in a fellow member of the species. The highly social species of birds such as the jackdaw or the grey-leg goose, have a complicated code of such signals which are uttered and understood by every bird without any previous experience. The perfect coordination of social behavior which is brought about by these actions and reactions conveys to the human observer the impression that the birds are talking and understanding a language of their own. Of course, this purely innate signal code of an animal species differs fundamentally from human language, every word of which must be learned laboriously by the human child. Moreover, being a genetically fixed character of the species-just as much as any bodily character-this so-called language is, for every individual animal species, ubiquitous in its distribution. Obvious though this fact may seem, it was, nevertheless, with something akin to naive surprise that I heard the jackdaws in northern Russia "talk" exactly the same, familiar "dialect" as my birds at home in Altenberg. The superficial similarity between these animal utterances and human languages diminishes further as it becomes gradually clear to the observer that the animal, in all these sounds and movements expressing its emotions, has in no way the conscious intention of influencing a fellow member of its species. This is proved by the fact that even geese or jackdaws reared and kept singly make all these signals as soon as the corresponding mood overtakes them. Under these circumstances the automatic and even mechanical character of these signals becomes strikingly apparent and reveals them as entirely different from human words.
The author wants to emphasize that（） A. the difference between human words and animal utterances is more than superficial
B. it is nothing but a conscious behavior for a bird to make meaningful sound
C. all species have their own means of communication such as body language
D. animal languages can be so complicated that they are apparently beyond human knowledge