Sentence:  I would draw a distinction between "transplants" and "cross-fertilisation". ... Cross-fertilisation implies a different, more indirect process. It implies that an external stimulus promotes an evolution within the receiving legal system. The evolution involves an internal adaptation by the receiving legal system in its own way. The new development is a distinctive but organic product of that system rather than a <WSD>bolt-on</WSD>. 
Meaning of the word marked with <WSD> tags:  An accessory that can be bolted on or otherwise attached.