That's true, Norm, and I've usually abided by that principle. In this case, I'm just not sure if it can work as is and would welcome guidance from a more able scholar such as yourself. If I was to submit this somewhere, it would be judged at a standard closer to yours than mine. I don't know if nas' suggestion, and your concurrence, is prompted by a liking for brevity, which goes against the form, or because it doesn't read well as is. 

Terence

Thanks for your thoughts. 

I found it interesting that Darwin said 
"When the principles of breeding and of inheritance are better understood, we shall not hear ignorant members of our legislature rejecting with scorn a plan for ascertaining by an easy method whether or not consanguineous marriages are injurious to man."

Yet he and his cousin, Emma Wedgwood, had 10 children, three of which died during childhood. Six of the surviving seven went on to have long-term marriages, but three of those marriages bore no children, suggesting that his children suffered from infertility. (That makes my title somewhat cutting.)

As you may know, his book was titled "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life". That's partly why I included "means of" and also to have the extra long line for the Clerihew.

I struggled to decide if that 4th line made sense with those words in it. If you think it makes more sense without them, I will happily change it.

Terence

I didn't know he'd married his first cousin. It still happens quite often in Muslim families.  Two of my work colleagues are married to their cousins and both have children.

You could trim down the last line to read

"influenced by unnatural selection"