Sentence:  A writer may tell me that he thinks man will ultimately become an ostrich. I cannot properly contradict him. But before he can expect to bring any reasonable person over to his opinion, he ought to shew, that the necks of mankind have been gradually <WSD>elongating</WSD> ... 
Meaning of the word marked with <WSD> tags:  To become long or longer by being pulled or stretched; to become elongated.