You need it to be impossible if you want to ensure you only eliminate species you're targeting instead of that species and random selection of others.
Life finds a way :-P
Ugh. You joke, but I loathe that sentence. Absolutely loathe it. The version in Crichton's book wasn't as bad as the tripe Ian Malcom spouted in the movie, but it's had a toxic influence on pop culture understandings of science and genetics.
More detail would be helpful. What was Chrichton's version? Why has this movie line been bad for popular understanding?