What reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction?" What is the meaning of the word concur as used in this sentence?

In this sentence, the meaning of the word 'concur' is "to act together to a common end or single effect" (one definition from the Merriam-Webster Dictionary).

Explanation:

To concur in reproduction would mean to work together, as opposed to the traditional meaning of concur ("to agree").